Manufacturing method of semiconductor device

ABSTRACT

In an embodiment, an insulating film is formed over a flat surface; a mask is formed over the insulating film; a slimming process is performed on the mask; an etching process is performed on the insulating film using the mask; a conductive film covering the insulating film is formed; a polishing process is performed on the conductive film and the insulating film, so that the conductive film and the insulating film have equal thicknesses; the conductive film is etched, so that a source electrode and a drain electrode which are thinner than the conductive film are formed; an oxide semiconductor film is formed in contact with the insulating film, the source electrode, and the drain electrode; a gate insulating film covering the oxide semiconductor film is formed; and a gate electrode is formed in a region which is over the gate insulating film and overlaps with the insulating film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The technical field of the present invention relates to a semiconductordevice and a manufacturing method of the semiconductor device. Here,semiconductor devices refer to general elements and devices whichfunction utilizing semiconductor characteristics.

2. Description of the Related Art

There are a wide variety of metal oxides and such metal oxides are usedfor various applications. Indium oxide is a well-known material and hasbeen used for transparent electrodes required in liquid crystal displaydevices or the like.

Some metal oxides have semiconductor characteristics. Examples of suchmetal oxides having semiconductor characteristics are tungsten oxide,tin oxide, indium oxide, zinc oxide, and the like. Thin film transistorsin which a channel formation region is formed using such metal oxidesare already known (e.g., see Patent Documents 1 to 4, Non-PatentDocument 1, and the like).

As metal oxides, not only single-component oxides but alsomulti-component oxides are known. For example, InGaO₃(ZnO)_(m) (m:natural number) having a homologous phase is known as a multi-componentoxide semiconductor including In, Ga, and Zn (e.g., see Non-PatentDocuments 2 to 4 and the like).

Furthermore, it has been proved that an oxide semiconductor includingsuch an In—Ga—Zn-based oxide is applicable to a channel formation regionof a thin film transistor (e.g., see Patent Document 5, Non-PatentDocuments 5 and 6, and the like).

In order to achieve high speed operation of a transistor or the like,miniaturization of the transistor is needed. For example, in PatentDocument 6, a thin film transistor including an oxide semiconductor usedfor a channel layer with a thickness of about 10 nm is disclosed. InNon-Patent Document 7, a thin film transistor including an oxidesemiconductor whose channel length is more than or equal to 2 μm andless than or equal to 100 μm is disclosed.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    S60-198861-   [Patent Document 2] Japanese Published Patent Application No.    H8-264794-   [Patent Document 3] Japanese Translation of PCT International    Application No. H11-505377-   [Patent Document 4] Japanese Published Patent Application No.    2000-150900-   [Patent Document 5] Japanese Published Patent Application No.    2004-103957-   [Patent Document 6] Japanese Published Patent Application No.    2010-21170

[Non-Patent Document]

-   [Non-Patent Document 1] M. W. Prins, K. O. Grosse-Holz, G    Muller, J. F. M. Cillessen, J. B. Giesbers, R. P. Weening, and R. M.    Wolf, “A ferroelectric transparent thin-film transistor,” Appl.    Phys. Lett., 17 June 1996, Vol. 68, pp. 3650-3652-   [Non-Patent Document 2] M. Nakamura, N. Kimizuka, and T. Mohri, “The    Phase Relations in the In₂O₃—Ga₂ZnO₄—ZnO System at 1350° C,” J.    Solid State Chem., 1991, Vol. 93, pp. 298-315-   [Non-Patent Document 3] N. Kimizuka, M. Isobe, and M. Nakamura,    “Syntheses and Single-Crystal Data of Homologous Compounds,    In₂O₃(ZnO)_(m) (m=3, 4, and 5), InGaO₃(ZnO)₃, and Ga₂O₃(ZnO)_(m)    (m=7, 8, 9, and 16) in the In₂O₃—ZnGa₂O₄—ZnO System,” J. Solid State    Chem., 1995, Vol. 116, pp. 170-178-   [Non-Patent Document 4] M. Nakamura, N. Kimizuka, T. Mohri, and M.    Isobe, “Syntheses and crystal structures of new homologous    compounds, indium iron zinc oxides (InFeO₃(ZnO)_(m) (m: natural    number) and related compounds,” KOTAI BUTSURI (SOLID STATE PHYSICS),    1993, Vol. 28, No. 5, pp. 317-327-   [Non-Patent Document 5] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M.    Hirano, and H. Hosono, “Thin-film transistor fabricated in    single-crystalline transparent oxide semiconductor,” SCIENCE, 2003,    Vol. 300, pp. 1269-1272-   [Non-Patent Document 6] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M.    Hirano, and H. Hosono, “Room-temperature fabrication of transparent    flexible thin-film transistors using amorphous oxide    semiconductors,” NATURE, 2004, Vol. 432, pp. 488-492-   [Non-Patent Document 7] T. Kawamura, H. Uchiyama, S. Saito, H.    Wakana, T. Mine, and M. Hatano, “Low-Voltage Operating Amorphous    Oxide TFTs,” IDW'09, pp. 1689-1692

SUMMARY OF THE INVENTION

In order to make use of characteristics of an oxide semiconductor suchas a light-transmitting property or an advantage that a transistorincluding an oxide semiconductor can easily have a large size, a glasssubstrate or the like is sometimes used as a substrate over which atransistor including an oxide semiconductor is formed. However, thedifference in height of a surface of the glass substrate is aboutseveral tens of micrometers at a maximum, which is large as compared tothe case of a silicon wafer or the like. Therefore, the accuracy ofphotomask projection exposure in semiconductor processing in the case ofusing a glass substrate is lower than that in the case of using asilicon wafer or the like, and the actual minimum feature size in thecase of using a glass substrate is several micrometers at most.

In the case of using an exposure apparatus for glass substrates (alsoreferred to as a stepper), the above-described limit of the feature sizemakes it difficult to achieve sufficient miniaturization at submicronlevel. However, in order to fully utilize characteristics of an oxidesemiconductor or an advantage attributed to the characteristics (e.g.,an advantage of easily realizing large size), miniaturization iscritical even under such a difficult situation.

In the case where a transistor is miniaturized, a defect generated inthe manufacturing process becomes a major problem. For example, in atransistor where a semiconductor film is formed over a wiringfunctioning as a source or drain electrode, a gate electrode or thelike, the wiring has a larger thickness than the semiconductor film,which causes poor coverage with the semiconductor film when thethickness of the semiconductor film is reduced along withminiaturization. As a result, disconnection, a defective connection, orthe like may occur.

Further, in the case where a transistor is miniaturized, another problemof a short channel effect arises. The short channel effect refers todegradation of electric characteristics which becomes obvious along withminiaturization of a transistor (a decrease in channel length (L)). Theshort channel effect results from the effect of an electric field of adrain on a source. Specific examples of the short channel effect are adecrease in threshold voltage, an increase in S value (subthresholdswing), an increase in leakage current, and the like. The short channeleffect is likely to occur particularly in a transistor including anoxide semiconductor because the threshold voltage of such a transistorcannot be controlled by doping, unlike a transistor including silicon.

In view of these problems, it is an object of one embodiment of theinvention disclosed herein to provide a semiconductor device whichsuppresses a defect and achieves miniaturization. Further, it is anotherobject of one embodiment of the invention disclosed herein to provide asemiconductor device which achieves miniaturization with favorablecharacteristics maintained.

An embodiment of the invention disclosed herein is a manufacturingmethod of a semiconductor device, which includes the steps of forming afirst insulating film over a flat surface; forming a first mask over thefirst insulating film; performing a slimming process on the first mask,so that a second mask is formed; performing an etching process on thefirst insulating film using the second mask, so that a second insulatingfilm is formed; forming a first conductive film covering the secondinsulating film; performing a polishing process on the first conductivefilm and the second insulating film, so that a second conductive filmand a third insulating film having equal thicknesses are formed; etchingthe second conductive film, so that a source electrode and a drainelectrode which are thinner than the second conductive film are formed;forming an oxide semiconductor film in contact with a part of a surfaceof the third insulating film, a part of a surface of the sourceelectrode, and a part of a surface of the drain electrode; forming agate insulating film covering the oxide semiconductor film; and forminga gate electrode in a region which is over the gate insulating film andoverlaps with the third insulating film. In the above-describedstructure, there is a case in which the source electrode and the drainelectrode are formed thinner than the third insulating film by 5 nm ormore and 20 nm or less.

Another embodiment of the invention disclosed herein is a manufacturingmethod of a semiconductor device, which includes the steps of forming afirst insulating film over a flat surface; forming a first mask over thefirst insulating film; performing a slimming process on the first mask,so that a second mask is formed; performing an etching process on thefirst insulating film using the second mask, so that a second insulatingfilm is formed; forming a first conductive film covering the secondinsulating film; performing a polishing process on the first conductivefilm and the second insulating film, so that a second conductive filmand a third insulating film having equal thicknesses are formed; etchingthe second conductive film and the third insulating film, so that afourth insulating film whose corner is removed and a source electrodeand a drain electrode which are thinner than the second conductive filmand the fourth insulating film are formed; forming an oxidesemiconductor film in contact with a part of a surface of the fourthinsulating film, a part of a surface of the source electrode, and a partof a surface of the drain electrode; forming a gate insulating filmcovering the oxide semiconductor film; and forming a gate electrode in aregion which is over the gate insulating film and overlaps with thefourth insulating film. In the above-described structure, there is acase in which the source electrode and the drain electrode are formedthinner than the fourth insulating film by 5 nm or more and 20 nm orless.

Another embodiment of the invention disclosed herein is a manufacturingmethod of a semiconductor device, which includes the steps of forming afirst transistor including a channel formation region, a first gateinsulating film over the channel formation region, a first gateelectrode which is over the first gate insulating film and overlaps withthe channel formation region, and a first source electrode and a firstdrain electrode which are electrically connected to the channelformation region; forming an interlayer insulating film covering thefirst transistor and having a flat surface; forming a first insulatingfilm over the interlayer insulating film; forming a first mask over thefirst insulating film; performing a slimming process on the first mask,so that a second mask is formed; performing an etching process on thefirst insulating film using the second mask, so that a second insulatingfilm is formed; forming a first conductive film covering the secondinsulating film; performing a polishing process on the first conductivefilm and the second insulating film, so that a second conductive filmand a third insulating film having equal thicknesses are formed; etchingthe second conductive film to form a second source electrode and asecond drain electrode which are thinner than the second conductivefilm; forming an oxide semiconductor film in contact with a part of asurface of the third insulating film, a part of a surface of the secondsource electrode, and a part of a surface of the second drain electrode;forming a second gate insulating film covering the oxide semiconductorfilm; and forming a second gate electrode in a region which is over thesecond gate insulating film and overlaps with the third insulating film.In the above-described structure, there is a case in which the secondsource electrode and the second drain electrode are formed thinner thanthe third insulating film by 5 nm or more and 20 nm or less.

Another embodiment of the invention disclosed herein is a manufacturingmethod of a semiconductor device, which includes the steps of forming afirst transistor including a channel formation region, a first gateinsulating film over the channel formation region, a first gateelectrode which is over the first gate insulating film and overlaps withthe channel formation region, and a first source electrode and a firstdrain electrode which are electrically connected to the channelformation region; forming an interlayer insulating film covering thefirst transistor and having a flat surface; forming a first insulatingfilm over the interlayer insulating film; forming a first mask over thefirst insulating film; performing a slimming process on the first mask,so that a second mask is formed; performing an etching process on thefirst insulating film using the second mask, so that a second insulatingfilm is formed; forming a first conductive film covering the secondinsulating film; performing a polishing process on the first conductivefilm and the second insulating film, so that a second conductive filmand a third insulating film having equal thicknesses are formed; etchingthe second conductive film and the third insulating film to form afourth insulating film whose corner is removed and a second sourceelectrode and a second drain electrode which are thinner than the secondconductive film and the fourth insulating film; forming an oxidesemiconductor film in contact with a part of a surface of the fourthinsulating film, a part of a surface of the second source electrode, anda part of a surface of the second drain electrode; forming a second gateinsulating film covering the oxide semiconductor film; and forming asecond gate electrode in a region which is over the second gateinsulating film and overlaps with the fourth insulating film. In theabove-described structure, there is a case in which the second sourceelectrode and the second drain electrode are formed thinner than thefourth insulating film by 5 nm or more and 20 nm or less.

In the above-described manufacturing method, there is a case in whichthe third insulating film has a thickness substantially equal to athickness of the second insulating film. Further, there is a case inwhich the flat surface has a root-mean-square roughness of less than orequal to 1 nm Further, there is a case in which the slimming process isperformed using an ashing process using an oxygen radical. Further,there is a case in which the polishing process is performed usingchemical mechanical polishing. Furthermore, there is a case in which thefirst insulating film is formed by sputtering.

In this specification and the like, the word concerning the thickness“substantially equal” means “almost equal” as well as “completelyequal.” For example, “substantially equal” refers to a case where, ascompared to the “completely equal” thickness situation, there is adifference in thickness that causes a negligible influence onsemiconductor device characteristics (a case where the influence oncharacteristics is 5% or less), a case where the thickness is slightlyreduced by polishing without intention (a case where the polishingamount is less than approximately 5 nm), and the like.

Note that in this specification and the like, the root-mean-square (RMS)roughness is obtained by three-dimensionally expanding the RMS roughnessof a cross section curve so as to be able to apply it to the measurementsurface. The RMS roughness is represented by the square root of the meanvalue of the square of the deviation from the reference surface to thespecific surface, and is obtained by the following formula.

$\begin{matrix}{R_{ms} = \sqrt{\frac{1}{S_{0}}{\int_{Y_{1}}^{Y_{2}}{\int_{X_{1}}^{X_{2}}{\left\{ {{F\left( {X,Y} \right)} - Z_{0}} \right\}^{2}\ {X}\ {Y}}}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Note that the measurement surface is a surface which is shown by all themeasurement data, and is represented by the following formula.

Z=F(X,Y)   [Formula 2]

The specific surface is a surface which is a target of roughnessmeasurement, and is a rectangular region which is surrounded by fourpoints represented by the coordinates (X₁, Y₁), (X₁, Y₂), (X₂, Y₁), and(X₂, Y₂). The area of the specific surface when the specific surface isflat ideally is denoted by S₀. Note that S₀ can be obtained by thefollowing formula.

S₀ =|X ₂ −X ₁ |·|Y ₂ −Y ₁|  [Formula 3]

In addition, the reference surface refers to a surface parallel to anX-Y surface at the average height of the specific surface. In short,when the average value of the height of the specific surface is denotedby Z₀, the height of the reference surface is also denoted by Z₀. Notethat Z₀ can be obtained by the following formula.

$\begin{matrix}{Z_{0} = {\frac{1}{S_{0}}{\int_{Y_{1}}^{Y_{2}}{\int_{X_{1}}^{X_{2}}{{F\left( {X,Y} \right)}\ {X}\ {Y}}}}}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack\end{matrix}$

Note that in this specification and the like, the root-mean-square (RMS)roughness is calculated in a region of 10 nm×10 nm, preferably 100nm×100 nm, further preferably 1 μm×1 μm from an AFM image obtained usingan atomic force microscope (AFM).

The channel length L of the above-described transistor is preferablygreater than or equal to 30 nm and less than 2000 nm (2 μm), furtherpreferably greater than or equal to 50 nm and less than or equal to 350nm, and still further preferably greater than or equal to 100 nm andless than or equal to 150 nm. The thickness of the oxide semiconductorfilm is preferably greater than or equal to 1 nm and less than or equalto 100 nm, further preferably greater than or equal to 2 nm and lessthan or equal to 50 nm, and still further preferably greater than orequal to 3 nm and less than or equal to 30 nm This can realize asemiconductor device which operates at high speed and consumes lowpower. In addition, the gate insulating film is preferably formed usinga high dielectric constant material such as hafnium oxide. Hafnium oxidehas a relative permittivity of about 15, which is much higher than thatof silicon oxide which is higher than or equal to 3 and lower than orequal to 4. Such a material facilitates miniaturization of asemiconductor device. Further, the oxide semiconductor film includes anintrinsic oxide semiconductor which is obtained by purification. Thisenables the carrier density of the oxide semiconductor film to be lowerthan 1×10¹²/cm³, preferably lower than 1.45×10¹⁰/cm³, and the off-statecurrent of the transistor to be smaller than or equal to 100 zA per unitchannel width (1 μm) (1 zA (zeptoampere) is 1×10⁻²¹ A), preferablysmaller than or equal to 10 zA per unit channel width (1 μm), forexample. The lowest off-state current is 1×10⁻³⁰ A per unit channelwidth (1 μm). That is, the value of the off-state current is at least1×10⁻³° A per unit channel width (1 μm) (that is, larger than or equalto 1×10⁻³⁰ A and smaller than or equal to 1×10⁻²¹ A per unit channelwidth (1 μm)). The gate electrode may overlap with a source electrodeand a drain electrode, or only an end portion and another end portion ofthe gate electrode may correspond to an end portion of a sourceelectrode and an end portion of a drain electrode, respectively. Here,the expression “to correspond” means “to overlap in the plane layout.”

In this specification, the semiconductor device indicates all thedevices that can operate by utilizing semiconductor characteristics. Forexample, a display device, a memory device, an integrated circuit, andthe like are included in the category of the semiconductor device.

Note that in this specification and the like, the term such as “over” or“below” does not necessarily mean that a component is placed “directlyon” or “directly under” another component. For example, the expression“a gate electrode over a gate insulating film” can mean the case wherethere is an additional component between the gate insulating film andthe gate electrode.

In addition, in this specification and the like, the term such as“electrode” or “wiring” does not limit the function of the component.For example, an “electrode” can be used as part of “wiring”, and viceversa. Further, the term “electrode” or “wiring” can also mean acombination of a plurality of “electrodes” and “wirings” formed in anintegrated manner.

Functions of a “source” and a “drain” are sometimes replaced with eachother when a transistor of opposite polarity is used or when thedirection of current flow is changed in circuit operation, for example.Therefore, the terms “source” and “drain” can be used to denote thedrain and the source, respectively, in this specification.

Note that in this specification and the like, the term “electricallyconnected” includes the case where components are connected through an“object having any electric function.” There is no particular limitationon the “object having any electric function” as long as electric signalscan be transmitted and received between components that are connectedthrough the object. Examples of the “object having any electricfunction” are a switching element such as a transistor, a resistor, aninductor, a capacitor, and an element with a variety of functions aswell as an electrode and a wiring.

In an embodiment of the invention disclosed herein, the transistor canbe sufficiently miniaturized regardless of the kind of substrate.Therefore, it is possible to provide a semiconductor device which fullyutilizes characteristics of an oxide semiconductor or an advantageattributed to the characteristics (e.g., an advantage of easilyrealizing large size).

An embodiment of the invention disclosed herein can solve a problemwhich may be caused along with the miniaturization. As a result, itbecomes possible to reduce the size of a transistor with favorablecharacteristics maintained.

By sufficiently reducing the size of the transistor, the area of asemiconductor device including the transistor is decreased, and thenumber of semiconductor devices obtained from one substrate isincreased. Accordingly, the manufacturing cost per semiconductor deviceis reduced. Further, since the size of a semiconductor device isreduced, a semiconductor device having higher function than aconventional semiconductor device in almost the same size can berealized. Moreover, advantageous effects such as high-speed operationand low power consumption due to a decrease in channel length can beobtained. In other words, according to one embodiment of the inventiondisclosed herein, miniaturization of a transistor including an oxidesemiconductor is achieved, and therefore a variety of advantageouseffects accompanied by the miniaturization can be obtained.

As described above, according to one embodiment of the inventiondisclosed herein, a semiconductor device which achieves miniaturizationas well as suppressing a defect or maintaining favorable characteristicscan be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are cross-sectional view each illustrating an example ofthe structure of a semiconductor device;

FIGS. 2A to 2H are cross-sectional views illustrating a manufacturingprocess of a semiconductor device;

FIG. 3A, FIG. 3B, and FIG. 3C are a cross-sectional view, a plan view,and a circuit diagram, respectively, of an example of the structure of asemiconductor device;

FIGS. 4A to 4D are cross-sectional views illustrating a manufacturingprocess of a semiconductor device;

FIGS. 5A to 5C are cross-sectional views illustrating a manufacturingprocess of a semiconductor device;

FIG. 6A1, FIG. 6A2, and FIG. 6B show application examples of asemiconductor device;

FIGS. 7A and 7B show application examples of a semiconductor device;

FIGS. 8A to 8C show an application example of a semiconductor device;

FIG. 9 shows an application example of a semiconductor device;

FIGS. 10A and 10B show an application example of a semiconductor device;

FIGS. 11A to 11F each illustrate an electronic device including asemiconductor device;

FIGS. 12A and 12B illustrate models which were used for simulation;

FIG. 13 shows a relation between the gate voltage V_(G) (V) and thedrain current I_(D) (A);

FIG. 14 shows a relation between the channel length L (nm) and thethreshold voltage V_(th) (V);

FIG. 15 shows a relation between the channel length L (nm) and the Svalue (V/dec);

FIGS. 16A and 16B illustrate models which were used for simulation;

FIG. 17 shows a relation between the gate voltage V_(G) (V) and thedrain current ID (A);

FIG. 18 illustrates a model which was used for simulation; and

FIG. 19 shows a relation between the gate voltage V_(G) (V) and thedrain current I_(D) (A).

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings. Note that the present invention is notlimited to the following description and it will be easily understood bythose skilled in the art that modes and details can be modified invarious ways without departing from the spirit and the scope of thepresent invention. Therefore, the present invention should not beconstrued as being limited to the description of the followingembodiments.

Note that the position, the size, the range, or the like of eachstructure illustrated in drawings and the like is not accuratelyrepresented in some cases for easy understanding. Therefore, theinvention disclosed herein is not necessarily limited to such position,size, range, and the like disclosed in the drawings and the like.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Embodiment 1

In this embodiment, a structure and a manufacturing method of asemiconductor device according to an embodiment of the inventiondisclosed herein will be described with reference to FIGS. 1A and 1B andFIGS. 2A to 2H.

<Example of Structure of Semiconductor Device>

FIGS. 1A and 1B illustrate examples of the structure of a semiconductordevice. FIG 1A illustrates a first example of the structure, and FIG. 1Billustrates a second example of the structure.

A transistor 162 in FIG. 1A includes an insulating film 143 a over asubstrate 140 having a surface over which components are formed, asource electrode 142 c and a drain electrode 142 d having a smallerthickness than the insulating film 143 a, an oxide semiconductor film144 in contact with a part of a surface of the insulating film 143 a, apart of a surface of the source electrode 142 c, and a part of a surfaceof the drain electrode 142 d, a gate insulating film 146 covering theoxide semiconductor film 144, and a gate electrode 148 a over the gateinsulating film 146. Further, there is some cases in which thetransistor 162 includes an insulating film 150 and an insulating film152.

By using an oxide semiconductor in an active layer of the transistor asillustrated in FIG. 1A, favorable characteristics can be obtained.

Here, the surface of the substrate 140 over which components are formedand a region in contact with the oxide semiconductor film in the part ofthe surface of the insulating film 143 a (in particular, a regionparallel to the surface over which components are formed) preferablyhave a root-mean-square (RMS) roughness of 1 nm or less (furtherpreferably 0.5 nm or less). The difference in height between the part ofthe surface (the upper surface) of the insulating film 143 a and thepart of the surface (the upper surface) of the source electrode 142 c orthe difference in height between the part of the surface (the uppersurface) of the insulating film 143 a and the part of the surface (theupper surface) of the drain electrode 142 d is preferably more than orequal to 5 nm and less than or equal to 20 nm. In other words, thethickness of the source electrode 142 c and the drain electrode 142 d ispreferably smaller than that of the insulating film 143 a by more thanor equal to 5 nm and less than or equal to 20 nm (the thickness of theinsulating film 143 a is preferably larger than that of the sourceelectrode 142 c and the drain electrode 142 d by more than or equal to 5nm and less than or equal to 20 nm).

Thus, the insulating film 143 a, the source electrode 142 c, and thedrain electrode 142 d are provided over a highly flat region having aroot-mean-square (RMS) roughness of 1 nm or less. In addition, a channelformation region of the transistor 162 is provided over a highly flatregion having a root-mean-square (RMS) roughness of 1 nm or less. Thismakes it possible to prevent a problem such as a short channel effecteven under a situation where the transistor 162 is miniaturized;accordingly, the transistor 162 having favorable characteristics can beprovided.

The increase in flatness of the surface over which components are formedcan make the thickness distribution of the oxide semiconductor film 144uniform; therefore, characteristics of the transistor 162 can beimproved. In addition, a decrease in coverage which may be caused by anextremely large difference in height can be suppressed, and adisconnection or a defective connection of the oxide semiconductor film144 can be prevented.

By making a small difference in height (e.g., a difference of more thanor equal to 5 nm and less than or equal to 20 nm) between the part ofthe surface of the insulating film 143 a and the part of the surface ofthe source electrode 142 c or between the part of the surface of theinsulating film 143 a and the part of the surface of the drain electrode142 d as described above, the path of electric current is extended, anda high-resistant region is provided in a part of the oxide semiconductorfilm. This makes it possible to alleviate the concentration of anelectric field in the transistor 162 and to suppress a short channeleffect.

Here, the oxide semiconductor film 144 is preferably an oxidesemiconductor film which is purified by sufficiently removing animpurity such as hydrogen therefrom or by sufficiently supplying oxygenthereto. Specifically, the hydrogen concentration of the oxidesemiconductor film 144 is 5×10¹⁹ atoms/cm³ or less, preferably 5×10¹⁸atoms/cm³ or less, further preferably 5×10¹⁷ atoms/cm³ or less, forexample. Note that the above hydrogen concentration of the oxidesemiconductor film 144 is measured by secondary ion mass spectrometry(SIMS). In the oxide semiconductor film 144 in which hydrogen is reducedto a sufficiently low concentration so that the oxide semiconductor filmis purified and in which a defect level in an energy gap induced byoxygen deficiency is reduced by sufficiently supplying oxygen asdescribed above, the concentration of carriers induced by a donor suchas hydrogen is less than 1×10¹²/cm³, preferably less than 1×10¹¹/cm³,and further preferably less than 1.45×10¹⁰/cm³. In addition, forexample, the off-state current (per unit channel width (1 μm), here) atroom temperature (25° C.) is 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A) orsmaller, preferably 10 zA or smaller. In this manner, by using an i-type(intrinsic) or substantially i-type oxide semiconductor, the transistor162 which has extremely favorable off-state current characteristics canbe obtained.

Note that the lowest off-state current is 1×10⁻³⁰ A per unit channelwidth (1 μm) in theory. That is, it can be said that the value of theoff-state current per unit channel width (1 μm) is greater than or equalto 1×10⁻³⁰ A.

A transistor 262 in FIG. 1B has a structure similar to that of thetransistor 162. That is, the transistor 262 includes an insulating film243 b over a substrate 240 having a surface over which components areformed, a source electrode 242 c and a drain electrode 242 d having asmaller thickness than the insulating film 243 b , an oxidesemiconductor film 244 in contact with a part of a surface of theinsulating film 243 b , a part of a surface of the source electrode 242c , and a part of a surface of the drain electrode 242 d , a gateinsulating film 246 covering the oxide semiconductor film 244, and agate electrode 248 a over the gate insulating film 246. Further, thereis some cases in which the transistor 262 includes an insulating film250 and an insulating film 252.

Further, the surface of the substrate 240 over which components areformed and a region in contact with the oxide semiconductor film in thepart of the surface of the insulating film 243 b (in particular, aregion parallel to the surface over which components are formed)preferably have a root-mean-square (RMS) roughness of 1 nm or less(further preferably 0.5 nm or less). The difference in height betweenthe part of the surface (the upper surface) of the insulating film 243 band the part of the surface (the upper surface) of the source electrode242 c or the difference in height between the part of the surface (theupper surface) of the insulating film 243 b and the part of the surface(the upper surface) of the drain electrode 242 d is preferably more thanor equal to 5 nm and less than or equal to 20 nm. In other words, thethickness of the source electrode 242 c and the drain electrode 242 d ispreferably smaller than that of the insulating film 243 b by more thanor equal to 5 nm and less than or equal to 20 nm (the thickness of theinsulating film 243 b is preferably larger than that of the sourceelectrode 242 c and the drain electrode 242 d by more than or equal to 5nm and less than or equal to 20 nm).

One difference between the transistor 262 in FIG. 1B and the transistor162 in FIG. 1A is whether a corner of the insulating film (that is, theinsulating film 143 a or the insulating film 243 b) existing below thechannel formation region of the transistor is removed. That is, theinsulating film 143 a has a shape in which a corner remains in thetransistor 162 in FIG. 1A, whereas the insulating film 243 b has a shapewith a rounded corner (a shape from which a corner is removed) in thetransistor 262 in FIG. 1B.

The effects resulting from the structure of FIG. 1B are similar to thoseobtained in the case of the structure in FIG. 1A. In other words,because a channel formation region of the transistor 262 is providedover a highly flat region, a problem such as a short channel effect canbe prevented even under a situation where the transistor 262 isminiaturized; accordingly, the transistor 262 having favorablecharacteristics can be provided. The increase in planarity of thesurface over which components are formed can make the thicknessdistribution of the oxide semiconductor film 244 uniform; therefore,characteristics of the transistor 262 can be improved. Further, adecrease in coverage which may be caused by an extremely largedifference in height can be suppressed, and a disconnection or adefective connection of the oxide semiconductor film 244 can beprevented. Moreover, by making a small difference in height between theinsulating film and the source electrode (or the drain electrode) asdescribed above, a high-resistant region is provided in a part of theoxide semiconductor film; accordingly, the concentration of an electricfield in the transistor 262 can be alleviated, and a short channeleffect can be suppressed.

Furthermore, since the corner of the insulating film 243 b is removed inthe transistor 262, a disconnection or a defective connection of theoxide semiconductor film 244 can be prevented and, in addition, theconcentration of an electric field can be further alleviated.

<Example of Manufacturing Method of Semiconductor Device>

Next, an example of a manufacturing method of the semiconductor devicewill be described with reference to FIGS. 2A to 2H. Here, FIGS. 2A to 2Hillustrate an example of a manufacturing method of the transistor 162illustrated in FIG. 1A.

First, the insulating film 143 and a mask 141 a are formed over thesubstrate 140 over which components are formed (see FIG. 2A).

Although there is no particular limitation on a substrate which can beused as the substrate 140, it is necessary that the substrate 140 has atleast heat resistance high enough to withstand heat treatment to beperformed later. For example, the substrate may be a glass substrate, aceramic substrate, a quartz substrate, a sapphire substrate, or thelike. Alternatively, the substrate may be a single crystal semiconductorsubstrate or a polycrystalline semiconductor substrate of silicon,silicon carbide, or the like, a compound semiconductor substrate ofsilicon germanium or the like, an SOI substrate, or the like as long asthe substrate has an insulating surface. Still alternatively, thesubstrate may be any of these substrates provided with a semiconductorelement provided thereover. Further, a base film may be provided overthe substrate 140.

Note that the surface over which components are formed of the substrate140 is preferably a sufficiently flat surface. For example, the surfaceover which components are formed of the substrate 140 may have aroot-mean-square (RMS) roughness of 1 nm or less (preferably 0.5 nm orless). When the transistor 162 is formed over such a surface, thecharacteristics can be sufficiently improved. In the case where thesurface over which components are formed of the substrate 140 has poorflatness, it is desirable that the surface be subjected to a chemicalmechanical polishing (CMP) process, an etching process, or the like soas to have the above flatness. Note that, for the details of the CMPprocess, later description can be referred to.

The mask 141 a can be formed by a photolithography technique using amaterial such as a photoresist. For light exposure at the time offorming the mask 141 a, extreme ultraviolet light having a wavelength asshort as more than or equal to several nanometers and less than or equalto several tens of nanometers is preferably used. The resolution oflight exposure using extreme ultraviolet light is high and the depth offocus is large. Thus, the mask 141 a can have a fine pattern.

As long as it is possible to form the mask 141 a having a sufficientlyfine pattern, a different method such as an ink-jet method may be usedto form the mask 141 a. In this case, it is unnecessary to use aphotosensitive material such as a photoresist as a material of the mask141 a.

The insulating film 143 can be formed using an inorganic insulatingmaterial such as silicon oxide, silicon oxynitride, silicon nitride,aluminum oxide, or gallium oxide. It is particularly preferable that theinsulating film 143 be formed using silicon oxide because the oxidesemiconductor film 144 formed later is in contact with the insulatingfilm 143. Although there is no particular limitation on the formationmethod of the insulating film 143, in consideration of contact with theoxide semiconductor film 144, a method in which hydrogen is sufficientlyreduced is preferably employed. As an example of such a method, asputtering method and the like can be given. Needless to say, anotherdeposition method such as a plasma CVD method may be used.

Next, a slimming process is performed on the mask 141 a , so that a mask141 b having a finer pattern is formed (see FIG. 2B).

As the slimming process, an ashing process in which oxygen in a radicalstate (an oxygen radical) or the like is used can be employed, forexample. However, the slimming process is not limited to the aboveashing process as long as the mask 141 a can be processed into a finerpattern. Note that the channel length (L) of a transistor is determinedby the formed mask 141 b. Therefore, it is preferable to employ aprocess with high controllability as the slimming process.

As the result of the slimming process, the line width of the mask 141 bcan be reduced to a length of less than or equal to half of theresolution limit, preferably one third of the resolution limit. Forexample, the line width can become more than or equal to 30 nm and lessthan or equal to 2000 nm (2 μm), preferably more than or equal to 50 nmand less than or equal to 350 nm This enables further miniaturization ofthe transistor.

Next, the insulating film 143 is processed with the use of the mask 141b, so that the insulating film 143 a is formed (see FIG. 2C).

The processing of the insulating film 143 into the insulating film 143 acan be performed using an etching process, for example. Although eithera dry etching process or a wet etching process may be performed as theetching process, dry etching with high controllability is preferablyused for miniaturization. The etching process may be performed so thatthe insulating film 143 a has a tapered shape. The taper angle can be,for example, greater than or equal to 30° and less than or equal to 60°.

The channel length (L) of the transistor 162 is determined by theinsulating film 143 a formed in the above-described manner. The size ofthe insulating film 143 a is substantially the same as the mask 141 b,that is, more than or equal to 30 nm and less than 2000 nm (2 μm),preferably more than or equal to 50 nm and less than or equal to 350 nm

Next, the conductive film 142 is formed so as to cover the insulatingfilm 143 a (see FIG. 2D).

The conductive film 142 can be formed by a PVD method such as asputtering method, or a CVD method such as a plasma CVD method. As amaterial of the conductive film 142, an element selected from aluminum,chromium, copper, tantalum, titanium, molybdenum, and tungsten, an alloyincluding any of these elements as a component, or the like can be used.A material including one of manganese, magnesium, zirconium, beryllium,neodymium, or scandium or a combination of a plurality of these elementsmay be used.

The conductive film 142 may have a single-layer structure or a stackedstructure including two or more layers. For example, the conductive film142 may have a single-layer structure of a titanium film or a titaniumnitride film, a single-layer structure of an aluminum film includingsilicon, a two-layer structure in which a titanium film is stacked overan aluminum film, a two-layer structure in which a titanium film isstacked over a titanium nitride film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder, or the like.

The conductive film 142 may be formed using a conductive metal oxide. Asthe conductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zincoxide (ZnO), an indium oxide-tin oxide alloy (In₂O₃—SnO₂, which isabbreviated to ITO in some cases), an indium oxide-zinc oxide alloy(In₂O₃—ZnO), or any of these metal oxide materials including silicon orsilicon oxide can be used.

It should be noted that the conductive film 142 needs to be formedthicker than the insulating film 143 a in terms of a later polishingprocess.

Next, a polishing process is performed on the conductive film 142, sothat a conductive film 142 a and a conductive film 142 b having athickness substantially equal to the thickness of the insulating film143 a are formed (see FIG. 2E).

As the polishing process, a chemical mechanical polishing (CMP) processcan be employed. The CMP process is a method for planarizing a surfaceof an object with a combination of chemical and mechanical actions. Morespecifically, the CMP process is a method in which a polishing cloth isattached to a polishing stage, the polishing stage and an object areeach rotated or swung while a slurry (an abrasive) is supplied betweenthe object and the polishing cloth, and the surface of the object ispolished by a chemical reaction between the slurry and the surface ofthe object and by a mechanical polishing action of the polishing clothon the object.

Through the CMP process, the root-mean-square (RMS) roughness ofsurfaces of the conductive film 142 a and the conductive film 142 b canbe 1 nm or less (preferably 0.5 nm or less). In some cases, the surfaceof the insulating film 143 a may also be polished through the CMPprocess; in such a case, the root-mean-square (RMS) roughness of thesurface of the insulating film 143 a may also become 1 nm or less(preferably 0.5 nm or less). Needless to say, a polishing process forthe purpose of polishing the surface of the insulating film 143 a mayalso be employed.

The CMP process as the polishing process may be performed once or pluraltimes. When the CMP process is performed in plural times, it ispreferable that the first polishing step be performed at a highpolishing rate and be followed by a final polishing step at a lowpolishing rate. By performing polishing steps with different polishingrates in combination, the planarity of the surfaces of the conductivefilm 142 a, the conductive film 142 b , and the insulating film 143 acan be further increased.

As the polishing process, a process in which a CMP process and anetching process are combined may be employed as well.

By the polishing process under theses conditions, the flatness of thesurface over which the oxide semiconductor film 144 is formed later canbe increased, and characteristics of the transistor 162 can be improvedaccordingly.

Next, an etching process is performed on the conductive film 142 a andthe conductive film 142 b , so that the source electrode 142 c and thedrain electrode 142 d which are thinner than the insulating film 143 aare formed (see FIG. 2F).

As the etching process, either or both of a dry etching process and awet etching process can be employed.

Here, the thickness of the source electrode 142 c and the drainelectrode 142 d is preferably smaller than that of the insulating film143 a by 5 nm or more and 20 nm or less. Such a difference in thicknessbetween the insulating film 143 a and the source and drain electrodes142 c and 142 d generates a small difference in height between thesurface of the insulating film 143 a and the surface of the sourceelectrode 142 c or between the surface of the insulating film 143 a andthe surface of the drain electrode 142 d. As a result, the path ofelectric current in the oxide semiconductor film is extended by thedifference in height, and a high-resistant region is provided in a partof the oxide semiconductor film. This makes it possible to alleviate theconcentration of an electric field in the transistor 162 and to suppressa short channel effect.

In some cases, a corner of the insulating film 143 a may be removed bythe etching process. In the case where the corner of the insulating film143 a is removed, a disconnection or a defective connection of the oxidesemiconductor film 144 to be formed later can be prevented and, inaddition, the concentration of an electric field can be furtheralleviated.

Note that after the above-described etching process, another etchingprocess may be performed to process the source electrode 142 c and thedrain electrode 142 d into a desired pattern. Details of the etchingprocess for processing the pattern are similar to those of theabove-described etching process.

Next, the oxide semiconductor film 144 covering a part of the surface ofthe insulating film 143 a, a part of the surface of the source electrode142 c, and a part of the surface of the drain electrode 142 d is formedin contact with a part of the surface of the insulating film 143 a, apart of the surface of the source electrode 142 c, and a part of thesurface of the drain electrode 142 d, and then the gate insulating film146 is formed so as to cover the oxide semiconductor film 144 (see FIG.2G).

The oxide semiconductor film includes at least one element selected fromIn, Ga, Sn, and Zn. For example, a four-component metal oxide such as anIn—Sn—Ga—Zn—O-based oxide semiconductor, a three-component metal oxidesuch as an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-basedoxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, or a Sn—Al—Zn—O-based oxide semiconductor, atwo-component metal oxide such as an In—Zn—O-based oxide semiconductor,a Sn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxidesemiconductor, a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-basedoxide semiconductor, an In—Mg—O-based oxide semiconductor, or anIn—Ga—O-based oxide semiconductor, a single-component metal oxide suchas an In—O-based oxide semiconductor, a Sn—O-based oxide semiconductor,or a Zn—O-based oxide semiconductor, or the like can be used. The aboveoxide semiconductors may include an element other than In, Ga, Sn, andZn, for example, SiO₂.

For example, an In—Ga—Zn—O-based oxide semiconductor means an oxidesemiconductor containing indium (In), gallium (Ga), and zinc (Zn), andthere is no limitation on the composition ratio thereof.

In particular, an In—Ga—Zn—O-based oxide semiconductor material hassufficiently high resistance when there is no electric field and thusoff-state current can be sufficiently reduced. In addition, also havinghigh field-effect mobility, the In—Ga—Zn—O-based oxide semiconductormaterial is suitable as a semiconductor material used in a semiconductordevice.

As a typical example of the In—Ga—Zn—O-based oxide semiconductormaterial, an oxide semiconductor material represented by InGaO₃(ZnO)_(m)(m>0) is given. Using M instead of Ga, there is an oxide semiconductormaterial represented by InMO₃(ZnO)_(m) (m>0). Here, M denotes one ormore metal elements selected from gallium (Ga), aluminum (Al), iron(Fe), nickel (Ni), manganese (Mn), cobalt (Co), or the like. Forexample, M may be Ga, Ga and Al, Ga and Fe, Ga and Ni, Ga and Mn, Ga andCo, or the like. Note that the above-described compositions are derivedfrom the crystal structures that the oxide semiconductor material canhave, and are mere examples.

As a target for forming the oxide semiconductor film using a sputteringmethod, a target having a composition ratio of In:Ga:Zn=1:x:y (x is 0 ormore and y is more than or equal to 0.5 and less than or equal to 5) ispreferably used. For example, a metal oxide target having a compositionratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] or the like can be used.Furthermore, a metal oxide target having a composition ratio ofIn₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio], a metal oxide target having acomposition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:4 [molar ratio], or a metaloxide target having a composition ratio of In₂O₃: Ga₂O₃:ZnO=1:0:2 [molarratio] can also be used.

In the case where an In—Zn—O-based material is used as the oxidesemiconductor, a target used has a composition ratio of In:Zn=50:1 to1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio),preferably In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2in a molar ratio), further preferably In:Zn=15:1 to 1.5:1(In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in a target usedfor forming an In—Zn—O-based oxide semiconductor which has an atomicratio of In:Zn:O=X:Y:Z, the relation of Z>1.5X+Y is satisfied.

In this embodiment, the oxide semiconductor film 144 having an amorphousstructure is formed by a sputtering method with the use of anIn—Ga—Zn—O-based metal oxide target. The thickness ranges from 1 nm to100 nm, preferably from 2 nm to 50 nm, further preferably from 3 nm to30 nm.

The relative density of the metal oxide in the metal oxide target is 80%or more, preferably 95% or more, and further preferably 99.9% or more.The use of the metal oxide target having high relative density makes itpossible to form an oxide semiconductor film having a dense structure.

The atmosphere in which the oxide semiconductor film 144 is formed ispreferably a rare gas (typically, argon) atmosphere, an oxygenatmosphere, or a mixed atmosphere containing a rare gas (typically,argon) and oxygen. Specifically, it is preferable to use a high-puritygas atmosphere, for example, from which an impurity such as hydrogen,water, a hydroxyl group, or hydride is removed to a concentration of 1ppm or less (preferably, a concentration of 10 ppb or less).

In forming the oxide semiconductor film 144, for example, an object tobe processed is held in a treatment chamber that is maintained inreduced pressure, and the object is heated to a temperature higher thanor equal to 100° C. and lower than 550° C., preferably higher than orequal to 200° C. and lower than or equal to 400° C. Alternatively, thetemperature of an object to be processed in forming the oxidesemiconductor film 144 may be room temperature (25° C. ±10° C.). Then,moisture in the treatment chamber is removed, a sputtering gas fromwhich hydrogen, water, or the like has been removed is introduced, andthe above-described target is used; thus, the oxide semiconductor film144 is formed. By forming the oxide semiconductor film 144 while heatingthe object, an impurity in the oxide semiconductor film 144 can bereduced. Moreover, damage due to sputtering can be reduced. In order toremove the moisture in the treatment chamber, it is preferable to use anentrapment vacuum pump. For example, a cryopump, an ion pump, a titaniumsublimation pump, or the like can be used. A turbo molecular pumpprovided with a cold trap may be used. Since it is possible to removehydrogen, water, or the like from the treatment chamber by evacuatingthe treatment chamber with a cryopump or the like, the concentration ofan impurity in the oxide semiconductor film can be reduced.

For example, conditions for forming the oxide semiconductor film 144 canbe set as follows: the distance between the object to be processed andthe target is 170 mm, the pressure is 0.4 Pa, the direct current (DC)power is 0.5 kW, and the atmosphere is an oxygen (100% oxygen)atmosphere, an argon (100% argon) atmosphere, or a mixed atmosphere ofoxygen and argon. Note that a pulsed direct current (DC) power supply ispreferably used because powder substances (also referred to as particlesor dust) can be reduced and the film thickness can be made uniform. Thethickness of the oxide semiconductor film 144 is set in the range of 1nm to 100 nm, preferably 2 nm to 50 nm, further preferably 3 nm to 30nm. By employing a structure according to the invention disclosedherein, a short channel effect due to miniaturization can be suppressedeven in the case of using the oxide semiconductor film 144 having such athickness. Note that the appropriate thickness of the oxidesemiconductor film differs depending on the oxide semiconductor materialused, the intended use of the semiconductor device, or the like;therefore, the thickness can be determined as appropriate in accordancewith the material, the intended use, or the like. Note that a surfaceover which the oxide semiconductor film 144 is formed is sufficientlyplanarized in one embodiment of the invention disclosed herein.Therefore, even an oxide semiconductor film having a small thickness canbe favorably formed.

Note that before the oxide semiconductor film 144 is formed by asputtering method, reverse sputtering in which an argon gas isintroduced and plasma is generated may be performed so that a materialattached to a surface over which the oxide semiconductor film 144 is tobe formed (e.g., the surface of the insulating film 143 a) is removed.Here, the reverse sputtering is a method in which ions collide with asurface so that the surface is modified, in contrast to normalsputtering in which ions collide with a sputtering target. An example ofa method for making ions collide with a surface is a method in whichhigh-frequency voltage is applied to the surface side in an argonatmosphere so that plasma is generated near the object. Note that anatmosphere of nitrogen, helium, oxygen, or the like may be used insteadof an argon atmosphere.

After the oxide semiconductor film 144 is formed, heat treatment (firstheat treatment) is preferably performed on the oxide semiconductor film144. Through the first heat treatment, excess hydrogen (including wateror a hydroxyl group) in the oxide semiconductor film 144 can be removed,the structure of the oxide semiconductor film 144 can be ordered, and adefect level in an energy gap can be reduced. For example, thetemperature of the first heat treatment is set higher than or equal to300° C. and lower than 550° C., or higher than or equal to 400° C. andlower than or equal to 500° C.

For example, after an object to be processed is introduced into anelectric furnace including a resistance heater or the like, heattreatment can be performed at 450° C. for one hour in a nitrogenatmosphere. The oxide semiconductor film is not exposed to the airduring the heat treatment so that entry of water or hydrogen can beprevented.

The heat treatment apparatus is not limited to the electric furnace andmay be an apparatus for heating an object by thermal radiation orthermal conduction from a medium such as a heated gas. For example, arapid thermal annealing (RTA) apparatus such as a gas rapid thermalannealing (GRTA) apparatus or a lamp rapid thermal annealing (LRTA)apparatus can be used. The LRTA apparatus is an apparatus for heating anobject by radiation of light (an electromagnetic wave) emitted from alamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, acarbon arc lamp, a high pressure sodium lamp, or a high pressure mercurylamp. The GRTA apparatus is an apparatus for performing heat treatmentusing a high-temperature gas. As the gas, an inert gas that does notreact with an object by heat treatment, for example, nitrogen or a raregas such as argon, is used.

For example, as the first heat treatment, a GRTA process may beperformed as follows. The object to be processed is put in a heatedinert gas atmosphere, heated for several minutes, and taken out of theinert gas atmosphere. The GRTA process enables high-temperature heattreatment in a short time. Moreover, the GRTA process can be employedeven when the temperature exceeds the upper temperature limit of theobject. Note that the inert gas may be switched to a gas includingoxygen during the process. This is because a defect level in energy gapdue to oxygen deficiency can be reduced by performing the first heattreatment in an atmosphere containing oxygen.

Note that as the inert gas atmosphere, an atmosphere that containsnitrogen or a rare gas (e.g., helium, neon, or argon) as its maincomponent and does not contain water, hydrogen, or the like ispreferably used. For example, the purity of nitrogen or a rare gas suchas helium, neon, or argon introduced into a heat treatment apparatus isgreater than or equal to 6 N (99.9999%), preferably greater than orequal to 7 N (99.99999%) (that is, the concentration of the impuritiesis 1 ppm or less, preferably 0.1 ppm or less).

In any case, impurities are reduced by the first heat treatment so thatan i-type (intrinsic) or substantially i-type oxide semiconductor filmis obtained. Accordingly, a transistor having highly excellentcharacteristics can be realized.

The above heat treatment (the first heat treatment) can be referred toas dehydration treatment, dehydrogenation treatment, or the like becauseof its effect of removing hydrogen, water, or the like. The dehydrationtreatment or the dehydrogenation treatment can also be performed at thefollowing timing: after the formation of the oxide semiconductor film144, after the formation of the gate insulating film 146, after theformation of the gate electrode, or the like. Such dehydration treatmentor dehydrogenation treatment may be conducted only once or plural times.

After the oxide semiconductor film 144 is formed, the oxidesemiconductor film 144 may be processed into an island-shaped oxidesemiconductor film, for example, by etching. The etching of the oxidesemiconductor film may be performed either before the heat treatment orafter the heat treatment. Although dry etching is preferred in terms ofelement miniaturization, wet etching may be used. An etching gas and anetchant can be selected as appropriate depending on a material of thefilm to be etched.

The gate insulating film 146 can be formed by a CVD method, a sputteringmethod, or the like. The gate insulating film 146 is preferably formedso as to contain silicon oxide, silicon nitride, silicon oxynitride,aluminum oxide, gallium oxide, tantalum oxide, hafnium oxide, yttriumoxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate towhich nitrogen is added (HfSi_(x)O_(y)N_(z) (x>0, y>0, z>0)), hafniumaluminate to which nitrogen is added (HfAl_(x)O_(y)N_(z) (x>0, y>0,z>0)), or the like. The gate insulating film 146 may have a single-layerstructure or a stacked structure. There is no particular limitation onthe thickness of the gate insulating film 146; the thickness ispreferably small in order to ensure the operation of the transistor whenthe semiconductor device is miniaturized. For example, in the case ofusing silicon oxide, the thickness can be greater than or equal to 1 nmand less than or equal to 100 nm, preferably greater than or equal to 10nm and less than or equal to 50 nm

When the gate insulating film 146 is thin as described above, gateleakage due to a tunnel effect or the like becomes a problem. In orderto solve the problem of gate leakage, the gate insulating film 146 maybe formed using a high dielectric constant (high-k) material such ashafnium oxide, tantalum oxide, yttrium oxide, hafnium silicate(HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogen is added(HfSi_(x)O_(y)N_(z) (x>0, y>0, z>0)), or hafnium aluminate to whichnitrogen is added (HfAl_(x)O_(y)N_(z) (x>0, y>0, z>0)). The use of ahigh-k material for the gate insulating film 146 makes it possible toincrease the thickness in order to suppress gate leakage as well asensuring electric characteristics. For example, hafnium oxide ispreferable as a material of the gate insulating film because therelative permittivity of hafnium oxide is approximately 15, which ismuch higher than that of silicon oxide which is higher than or equal to3 and lower than or equal to 4. Note that a stacked structure of a filmincluding a high-k material and a film including any of silicon oxide,silicon nitride, silicon oxynitride, silicon nitride oxide, aluminumoxide, or the like may also be employed.

After the gate insulating film 146 is formed, second heat treatment ispreferably performed in an inert gas atmosphere or an oxygen atmosphere.The temperature of the heat treatment is set in the range of higher thanor equal to 200° C. and lower than or equal to 450° C., preferablyhigher than or equal to 250° C. and lower than or equal to 350° C. Forexample, the heat treatment may be performed at 250° C. for one hour ina nitrogen atmosphere. By the second heat treatment, variation inelectric characteristics of the transistor can be reduced. In the casewhere the gate insulating film 146 contains oxygen, oxygen can besupplied to the oxide semiconductor film 144 and oxygen vacancies in theoxide semiconductor film 144 can be filled; thus, the oxidesemiconductor film 144 which is i-type (intrinsic) or substantiallyi-type can be formed.

In this embodiment, the second heat treatment is performed after thegate insulating film 146 is formed; however, the timing of the secondheat treatment is not limited to this. For example, the second heattreatment may be performed after the gate electrode is formed.Alternatively, the first heat treatment and the second heat treatmentmay be performed in succession, or the first heat treatment may doubleas the second heat treatment, or the second heat treatment may double asthe first heat treatment.

By performing at least one of the first heat treatment and the secondheat treatment as described above, the oxide semiconductor film 144 canbe purified so as to contain impurities other than main componentsthereof as little as possible.

Next, the gate electrode 148 a is formed over the gate insulating film146. Then, an insulating film 150 and an insulating film 152 are formedso as to cover the gate insulating film 146, the gate electrode 148 a ,and the like (see FIG. 2H).

The gate electrode 148 a can be formed by forming a conductive film overthe gate insulating film 146 and then selectively etching the conductivefilm. The conductive film to be the gate electrode 148 a can be formedby a PVD method such as a sputtering method, or a CVD method such as aplasma CVD method. The details of the manufacturing method or thematerial are similar to those of the source electrode 142 c, the drainelectrode 142 d, or the like; thus, the description thereof can bereferred to. Note that although a part of the gate electrode 148 aoverlaps with the source electrode 142 c and the drain electrode 142 din the structure employed here, the invention disclosed herein is notlimited to this structure. It is possible to employ a structure in whichan end portion of the gate electrode 148 a and an end portion of thesource electrode 142 c overlap with each other, and an end portion ofthe gate electrode 148 a and an end portion of the drain electrode 142 doverlap with each other.

The insulating film 150 and the insulating film 152 can be formed by aPVD method, a CVD method, or the like. The insulating film 150 and theinsulating film 152 can be formed using an inorganic insulating materialsuch as silicon oxide, silicon oxynitride, silicon nitride, hafniumoxide, aluminum oxide, or gallium oxide.

Note that the insulating film 150 and the insulating film 152 arepreferably formed using a low dielectric constant material or to have astructure with low dielectric constant (e.g., a porous structure). Whenthe dielectric constant of the insulating film 150 and the insulatingfilm 152 is reduced, the capacitance generated between wirings orelectrodes can be reduced, which results in higher speed operation.

Note that, although a stacked structure of the insulating film 150 andthe insulating film 152 is employed in this embodiment, one embodimentof the invention disclosed herein is not limited thereto. A single-layerstructure or a stacked structure including three or more layers can alsobe used for the insulating film. Alternatively, the insulating film maybe omitted.

Note that the insulating film 152 is preferably formed so as to have aflat surface. By forming the insulating film 152 having a flat surface,an electrode, a wiring, or the like can be favorably formed over theinsulating film 152 even in the case where the semiconductor device isminiaturized, for example. The insulating film 152 can be planarized bya method such as a chemical mechanical polishing (CMP) process.

Through the above-described process, the transistor 162 including thepurified oxide semiconductor film 144 is completed (see FIG. 2H).

Note that after the above-described process, a variety of wirings,electrodes, or the like may be formed. The wirings or the electrodes canbe formed by a so-called damascene method or dual damascene method inwhich a conductive film is formed and processed, or the like.

In an embodiment of the invention disclosed herein, a slimming processis performed after formation of a mask in order to further miniaturizethe pattern of the mask in the above-described manner. This allows thetransistor to be sufficiently miniaturized regardless of the kind ofsubstrate. Therefore, it is possible to provide a semiconductor devicewhich fully utilizes an advantage of an oxide semiconductor.

In addition, the transistor 162 is provided over a highly flat regionwhose root-mean-square (RMS) roughness is 1 nm or less; therefore, aproblem such as a short channel effect can be prevented even under asituation where the transistor 162 is miniaturized and thus thetransistor 162 having favorable characteristics can be provided.

The increase in flatness of the surface over which components are formedcan make the thickness distribution of the oxide semiconductor film 144uniform; therefore, characteristics of the transistor 162 can beimproved. In addition, a decrease in coverage which may be caused by anextremely large difference in height can be suppressed, and adisconnection or a defective connection of the oxide semiconductor film144 can be prevented.

Moreover, the path of electric current can be extended and ahigh-resistant region is provided in a part of the oxide semiconductorfilm by making a small difference (e.g., a difference of more than orequal to 5 nm and less than or equal to 20 nm) in height between thepart of the surface of the insulating film 143 a and the part of thesurface of the source electrode 142 c or between the part of the surfaceof the insulating film 143 a and the part of the surface of the drainelectrode 142 d. This makes it possible to alleviate the concentrationof an electric field in the transistor 162 and to suppress a shortchannel effect.

In this manner, according to one embodiment of the invention disclosedherein, a semiconductor device which achieves miniaturization as well assuppressing a defect or maintaining favorable characteristics can beprovided.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 2

In this embodiment, a structure and a manufacturing method of asemiconductor device according to another embodiment of the inventiondisclosed herein will be described with reference to FIGS. 3A to 3C,FIGS. 4A to 4D, and FIGS. 5A to 5C

<Example of Structure of Semiconductor Device>

FIGS. 3A to 3C illustrate an example of a structure of a semiconductordevice. FIG. 3A is a cross-sectional view of the semiconductor device;FIG. 3B is a plan view of the semiconductor device; and FIG. 3Cillustrates a circuit configuration of the semiconductor device. Notethat a structure of the semiconductor device is mainly described in thisembodiment, and operation of the semiconductor device will be describedin detail in the following embodiment. Note that the semiconductordevice illustrated in FIGS. 3A to 3C is just an example of asemiconductor device having a predetermined function and does notexhaustively represent the semiconductor device of the inventiondisclosed herein. The semiconductor device according to the inventiondisclosed herein can have another function by changing connection of anelectrode or the like as appropriate.

FIG. 3A corresponds to a cross-sectional view along line A1-A2 and lineB1-B2 in FIG. 3B. The semiconductor device illustrated in FIGS. 3A and3B includes the transistor 162 described in the above embodiment, atransistor 160 below the transistor 162, and a capacitor 164.

Here, a semiconductor material of the transistor 162 and a semiconductormaterial of the transistor 160 are preferably different materials. Forexample, the semiconductor material of the transistor 162 may be anoxide semiconductor, and the semiconductor material of the transistor160 may be a semiconductor material (e.g., silicon) other than an oxidesemiconductor. A transistor including an oxide semiconductor can holdcharge for a long time owing to its characteristics. On the other hand,a transistor including a material other than an oxide semiconductor canoperate at high speed easily.

The transistor 160 in FIGS. 3A to 3C includes a channel formation region116 provided in a substrate 100 including a semiconductor material(e.g., silicon), impurity regions 120 provided so that the channelformation region 116 is sandwiched therebetween, metal compound regions124 in contact with the impurity regions 120, a gate insulating film 108provided over the channel formation region 116, and a gate electrode 110provided over the gate insulating film 108. Note that a transistor whosesource electrode and drain electrode are not illustrated in a drawingmay also be referred to as a transistor for the sake of convenience.Further, in such a case, in description of a connection of such atransistor, a source electrode and a drain electrode may be described inthis specification.

Further, an element isolation insulating film 106 is formed over thesubstrate 100 so as to surround the transistor 160, and an insulatingfilm 130 is formed to cover the transistor 160. Note that in order torealize higher integration, the transistor 160 preferably has astructure without a sidewall insulating film as illustrated in FIGS. 3Aand 3B. On the other hand, in the case where characteristics of thetransistor 160 have priority, a sidewall insulating film may be providedon a side surface of the gate electrode 110, and the impurity regions120 may include regions having different impurity concentrations.

The structure of the transistor 162 in FIGS. 3A to 3C is similar to thestructure of the transistor 162 in the above embodiment. Note that inthis embodiment, the source electrode 142 c (which may be the drainelectrode) of the transistor 162 is connected to the gate electrode 110of the transistor 160. Needless to say, the transistor 262 can also beused instead of the transistor 162.

The capacitor 164 in FIGS. 3A to 3C includes the source electrode 142 c(which may be the drain electrode), the oxide semiconductor film 144,the gate insulating film 146, and an electrode 148 b. In other words,the source electrode 142 c functions as one electrode of the capacitor164, and the electrode 148 b functions as the other electrode of thecapacitor 164. Note that the electrode 148 b is formed in a processsimilar to that of the gate electrode 148 a of the transistor 162.

Note that in the capacitor 164 of FIGS. 3A to 3C, the oxidesemiconductor film 144 and the gate insulating film 146 are stacked,whereby insulation between the source electrode 142 c and the electrode148 b can be sufficiently secured. It is needless to say that thecapacitor 164 without including the oxide semiconductor film 144 may beemployed in order to secure sufficient capacitance. In addition, in thecase where no capacitor is needed, a structure in which the capacitor164 is not provided is also possible.

In this embodiment, the transistor 162 and the capacitor 164 areprovided so as to overlap with the transistor 160. By employing such aplanar layout, higher integration can be realized. For example, giventhat the minimum feature size is F, the area occupied by thesemiconductor device can be more than or equal to 15 F² and less than orequal to 25 F².

Note that the structure of a semiconductor device according to theinvention disclosed herein is not limited to that illustrated in FIGS.3A to 3C. Since the technical idea of an embodiment of the inventiondisclosed herein is to form a stacked structure of an oxidesemiconductor and a semiconductor material other than an oxidesemiconductor, the details such as a connection relationship ofelectrodes can be changed as appropriate.

<Manufacturing Method of Semiconductor Device>

Next, an example of a manufacturing method of the semiconductor devicewill be described with reference to FIGS. 4A to 4D and FIGS. 5A to 5C. Amanufacturing method of the transistor 162 is similar to that in theabove embodiment; thus, a manufacturing method of the transistor 160will be mainly described here.

First, the substrate 100 including a semiconductor material is prepared(see FIG. 4A). A single crystal semiconductor substrate or apolycrystalline semiconductor substrate of silicon, silicon carbide, orthe like, a compound semiconductor substrate of silicon germanium or thelike, an SOI substrate, or the like can be used as the substrate 100including a semiconductor material. Here, an example of the case where asingle crystal silicon substrate is used as the substrate 100 includinga semiconductor material is described. Note that the term “SOIsubstrate” generally means a substrate where a silicon semiconductorfilm is provided over an insulating surface. In this specification andthe like, the term “SOI substrate” also means a substrate where asemiconductor film including a material other than silicon is providedover an insulating surface. That is, a semiconductor layer included inthe “SOI substrate” is not limited to a silicon semiconductor film.Moreover, the SOI substrate can be a substrate having a structure wherea semiconductor film is provided over an insulating substrate such as aglass substrate with an insulating film interposed therebetween.

It is preferable that a single crystal semiconductor substrate ofsilicon or the like be particularly used as the substrate 100 includinga semiconductor material because the speed of reading operation of thesemiconductor device can be increased.

Note that an impurity element may be added to a region which laterfunctions as the channel formation region 116 of the transistor 160, inorder to control the threshold voltage of the transistor. Here, animpurity element imparting conductivity is added so that the thresholdvoltage of the transistor 160 becomes positive. When the semiconductormaterial is silicon, the impurity imparting conductivity may be boron,aluminum, gallium, or the like. Note that it is preferable to performheat treatment after adding an impurity element, in order to activatethe impurity element or reduce defects which may be generated duringaddition of the impurity element.

Next, a protective layer 102 serving as a mask for forming an elementisolation insulating film is formed over the substrate 100 (see FIG.4A). As the protective layer 102, an insulating film including amaterial such as silicon oxide, silicon nitride, silicon oxynitride, orthe like can be used, for example.

Next, a part of the substrate 100 in a region not covered with theprotective layer 102 (i.e., in an exposed region) is removed by etchingusing the protective layer 102 as a mask. Thus, a semiconductor region104 isolated from other semiconductor regions is formed (see FIG. 4B).As the etching, dry etching is preferably performed, but wet etching maybe performed. An etching gas or an etchant can be selected asappropriate depending on a material to be etched. /

Then, an insulating film is formed so as to cover the semiconductorregion 104, and the insulating film in a region overlapping with thesemiconductor region 104 is selectively removed; thus, the elementisolation insulating film 106 is formed (see FIG. 4C). The insulatingfilm is formed using silicon oxide, silicon nitride, silicon oxynitride,or the like. As a method for removing the insulating film, any of anetching process, a polishing process such as a chemical mechanicalpolishing (CMP) process, and the like can be employed. Note that theprotective layer 102 is removed after the formation of the semiconductorregion 104 or after the formation of the element isolation insulatingfilm 106.

Next, an insulating film is formed over a surface of the semiconductorregion 104, and a layer including a conductive material is formed overthe insulating film.

The insulating film is processed into a gate insulating film later andcan be formed by heat treatment (thermal oxidation treatment, thermalnitridation treatment, or the like) of the surface of the semiconductorregion 104, for example. Instead of heat treatment, high-density plasmatreatment may be employed. The high-density plasma treatment can beperformed using, for example, a mixed gas of any of a rare gas such asHe, Ar, Kr, or Xe, oxygen, nitrogen oxide, ammonia, nitrogen, hydrogen,and the like. It is needless to say that the insulating film may beformed by a CVD method, a sputtering method, or the like. The insulatingfilm preferably has a single-layer structure or a stacked structureincluding silicon oxide, silicon oxynitride, silicon nitride, hafniumoxide, aluminum oxide, tantalum oxide, yttrium oxide, hafnium silicate(HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogen is added(HfSi_(x)O_(y)N_(z) (x>0, y>0, z>0)), hafnium aluminate to whichnitrogen is added (HfAl_(x)O_(y)N_(z) (x>0, y>0, z>0)), or the like. Theinsulating film can have a thickness of greater than or equal to 1 nmand less than or equal to 100 nm, preferably, greater than or equal to10 nm and less than or equal to 50 nm, for example.

The layer including a conductive material can be formed using a metalmaterial such as aluminum, copper, titanium, tantalum, or tungsten. Thelayer including a conductive material may be formed using asemiconductor material such as polycrystalline silicon. There is noparticular limitation on the formation method of the layer including aconductive material, and a variety of film formation methods such as anevaporation method, a CVD method, a sputtering method, or a spin coatingmethod can be employed. Note that this embodiment shows an example ofthe case where the layer including a conductive material is formed usinga metal material.

After that, the insulating film and the layer including a conductivematerial are selectively etched; thus, the gate insulating film 108 andthe gate electrode 110 are formed (see FIG. 4C).

Next, phosphorus (P), arsenic (As), or the like is added to thesemiconductor region 104, whereby the channel formation region 116 andthe impurity regions 120 are formed (see FIG. 4D). Note that phosphorusor arsenic is added here in order to form an n-channel transistor; animpurity element such as boron (B) or aluminum (Al) may be added in thecase of forming a p-channel transistor. Here, the concentration of theimpurity added can be set as appropriate; the concentration ispreferably set high when a semiconductor element is highly miniaturized.

Note that a sidewall insulating film may be formed around the gateelectrode 110, and impurity regions to which the impurity element isadded at a different concentration may be formed.

Next, a metal layer 122 is formed so as to cover the gate electrode 110,the impurity regions 120, and the like (see FIG. 5A). The metal layer122 can be formed by any of a variety of film formation methods such asa vacuum evaporation method, a sputtering method, and a spin coatingmethod. The metal layer 122 is preferably formed using a metal materialwhich forms a low-resistance metal compound by reacting with thesemiconductor material included in the semiconductor region 104.Examples of such a metal material are titanium, tantalum, tungsten,nickel, cobalt, platinum, and the like.

Next, heat treatment is performed so that the metal layer 122 reactswith the semiconductor material. Thus, the metal compound regions 124that are in contact with the impurity regions 120 are formed (see FIG.5A). Note that when the gate electrode 110 is formed usingpolycrystalline silicon or the like, a metal compound region is alsoformed in a portion of the gate electrode 110 which is in contact withthe metal layer 122.

As the heat treatment, irradiation with a flash lamp can be employed,for example. Although it is needless to say that another heat treatmentmethod may be used, a method by which heat treatment can be achieved inan extremely short time is preferably used in order to improve thecontrollability of chemical reaction for formation of the metalcompound. Note that the metal compound regions are formed by reaction ofthe metal material and the semiconductor material and have sufficientlyhigh conductivity. The formation of the metal compound regions canproperly reduce the electric resistance and improve elementcharacteristics. Note that the metal layer 122 is removed after themetal compound regions 124 are formed.

Next, the insulating film 130 is formed so as to cover the componentsformed in the above steps (see FIG. 5B). The insulating film 130 can beformed using an inorganic insulating material such as silicon oxide,silicon oxynitride, silicon nitride, or aluminum oxide. It isparticularly preferable to use a low dielectric constant (low-k)material for the insulating film 130 because capacitance due to overlapof electrodes or wirings can be sufficiently reduced. Note that a porousinsulating film including such a material may be employed as theinsulating film 130. The porous insulating film has a lower dielectricconstant than an insulating film with high density and thus makes itpossible to further reduce capacitance due to electrodes or wirings.Alternatively, the insulating film 130 can be formed using an organicinsulating material such as polyimide or acrylic. Note that although asingle-layer structure of the insulating film 130 is used in thisembodiment, an embodiment of the invention disclosed herein is notlimited to this example. A stacked structure including two or morelayers may be employed.

Through the above-described process, the transistor 160 is formed withthe use of the substrate 100 including a semiconductor material (seeFIG. 5B). A feature of the transistor 160 is that it can operate at highspeed. With the use of that transistor as a transistor for reading, datacan be read at high speed.

Then, as treatment performed before the transistor 162 and the capacitor164 are formed, a CMP process is performed on the insulating film 130 sothat an upper surface of the gate electrode 110 is exposed (see FIG.5C). As treatment for exposing the upper surface of the gate electrode110, an etching process or the like can also be employed instead of theCMP process; in order to improve characteristics of the transistor 162,a surface of the insulating film 130 is preferably made as flat aspossible. For example, the insulating film 130 is planarized so that theroot-mean-square (RMS) roughness of the surface of the insulating film130 becomes 1 nm or less. This makes it possible to improvecharacteristics of a semiconductor device formed over the insulatingfilm 130.

Note that before or after each of the above steps, a step of forming anelectrode, a wiring, a semiconductor film, an insulating film, or thelike may be further performed. For example, when the wiring has amulti-layer structure including a stack of insulating films andconductive films, a highly integrated semiconductor device can berealized.

Then, the transistor 162 and the capacitor 164 are formed; thus, thesemiconductor device is completed.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 3

In this embodiment, an example of application of a semiconductor deviceaccording to one embodiment of the invention disclosed herein will bedescribed with reference to FIGS. 6A-1, 6A-2, and 6B. Here, an exampleof a memory device will be described. Note that in some circuitdiagrams, “OS” is written beside a transistor in order to indicate thatthe transistor includes an oxide semiconductor.

In a semiconductor device which can be used as a memory device, which isillustrated in FIG. 6A1, a first wiring (1st Line) is electricallyconnected to a source electrode (or a drain electrode) of a transistor1000. A second wiring (2nd Line) is electrically connected to a drainelectrode (or a source electrode) of the transistor 1000. A third wiring(3rd Line) is electrically connected to a source electrode (or a drainelectrode) of a transistor 1010. A fourth wiring (4th Line) iselectrically connected to a gate electrode of the transistor 1010.Furthermore, a gate electrode of the transistor 1000 and the drainelectrode (or the source electrode) of the transistor 1010 areelectrically connected to one electrode of a capacitor 1020. A fifthwiring (5th Line) is electrically connected to the other electrode ofthe capacitor 1020.

Here, a transistor including an oxide semiconductor is used as thetransistor 1010. For example, the transistor described in the aboveembodiment can be used as the transistor including an oxidesemiconductor. A transistor including an oxide semiconductor has acharacteristic of a significantly small off-state current. For thatreason, a potential of the gate electrode of the transistor 1000 can beheld for an extremely long period by turning off the transistor 1010.Furthermore, with the use of the transistor described in the aboveembodiment, a short channel effect of the transistor 1010 can besuppressed, and miniaturization can be achieved. By providing thecapacitor 1020, holding of charge applied to the gate electrode of thetransistor 1000 and reading of the held data can be performed moreeasily. Here, the capacitor described in the above embodiment can beused as the capacitor 1020, for example.

In addition, a transistor including a semiconductor material other thanan oxide semiconductor is used as the transistor 1000. As thesemiconductor material other than an oxide semiconductor, for example,silicon, germanium, silicon germanium, silicon carbide, galliumarsenide, or the like can be used, and a single crystal semiconductor ispreferably used. Alternatively, an organic semiconductor material or thelike may be used. A transistor including such a semiconductor materialcan operate at high speed easily. Here, the transistor described in theabove embodiment can be used as the transistor including a semiconductormaterial other than an oxide semiconductor, for example.

Alternatively, a structure in which the capacitor 1020 is not providedis also possible as illustrated in FIG. 6B.

The semiconductor device in FIG. 6A1 utilizes an advantage that thepotential of the gate electrode of the transistor 1000 can be held, andcan thus write, hold, and read data as follows.

First of all, writing and holding of data will be described. First, thepotential of the fourth wiring is set to a potential at which thetransistor 1010 is on, so that the transistor 1010 is turned on.Accordingly, the potential of the third wiring is supplied to the gateelectrode of the transistor 1000 and the capacitor 1020. That is,predetermined charge is supplied to the gate electrode of the transistor1000 (writing). Here, one of two charges supplying different potentials(hereinafter, a charge supplying a low potential is referred to ascharge Q_(L) and a charge supplying a high potential is referred to ascharge Q_(H)) is given. Note that charges supplying three or moredifferent potentials may be applied in order to improve storagecapacity. After that, the potential of the fourth wiring is set to apotential at which the transistor 1010 is off, so that the transistor1010 is turned off. Thus, the charge supplied to the gate electrode ofthe transistor 1000 is held (holding).

Since the off-state current of the transistor 1010 is significantlysmall, the charge of the gate electrode of the transistor 1000 is heldfor a long time.

Next, reading of data will be described. By supplying an appropriatepotential (reading potential) to the fifth wiring while a predeterminedpotential (a constant potential) is supplied to the first wiring, thepotential of the second wiring varies depending on the amount of chargeheld in the gate electrode of the transistor 1000. This is generallybecause, when the transistor 1000 is an n-channel transistor, anapparent threshold voltage V_(th H) in the case where charge Q_(H) issupplied to the gate electrode of the transistor 1000 is lower than anapparent threshold voltage V_(th) _(—) _(L), in the case where chargeQ_(L) is supplied to the gate electrode of the transistor 1000. Here, anapparent threshold voltage refers to the potential of the fifth wiring,which is needed to turn on the transistor 1000. Thus, when the potentialof the fifth wiring is set to a potential V₀ that is intermediatepotential between apparent threshold voltages V_(th) _(—) _(H) andV_(th) _(—) _(L), charge supplied to the gate electrode of thetransistor 1000 can be determined For example, in the case where chargeQ_(H) is supplied in writing, when the potential of the fifth wiring isV₀ (>V_(th) _(—) _(H)), the transistor 1000 is turned on. In the casewhere charge Q_(L) is supplied in writing, even when the potential ofthe fifth wiring is V₀ (<V_(th) _(—) _(L)), the transistor 1000 remainsin an off state. Therefore, the held data can be read by measuring thepotential of the second wiring.

Note that in the case where memory cells are arrayed to be used, it isnecessary to read data only from an intended memory cell. In the casewhere data of a predetermined memory cell are read and data of the othermemory cells are not read, a potential at which the transistor 1000 isoff regardless of the state of the gate electrode, that is, a potentiallower than the apparent threshold voltage V_(th) _(—) _(H) may besupplied to fifth wirings of the memory cells that are not a target forreading. Alternatively, a potential at which the transistor 1000 is onregardless of the state of the gate electrode, that is, a potentialhigher than the apparent threshold voltage V_(th) _(—) _(L), may besupplied to the fifth wirings.

Next, rewriting of data will be described. Rewriting of data isperformed in a manner similar to that of the writing and holding ofdata. That is, the potential of the fourth wiring is set to a potentialat which the transistor 1010 is on, so that the transistor 1010 isturned on. Accordingly, the potential of the third wiring (a potentialfor new data) is supplied to the gate electrode of the transistor 1000and to the capacitor 1020. After that, the potential of the fourthwiring is set to a potential at which the transistor 1010 is off, sothat the transistor 1010 is turned off. Accordingly, charge for new datais supplied to the gate electrode of the transistor 1000.

In the semiconductor device according to the invention disclosed herein,data can be directly rewritten by another writing of data as describedabove. Therefore, extraction of charge from a floating gate with the useof a high voltage which is necessary in a flash memory or the like isnot needed, and thus a decrease in operation speed due to erasingoperation can be suppressed. In other words, high-speed operation of thesemiconductor device can be realized.

Note that the drain electrode (or the source electrode) of thetransistor 1010 is electrically connected to the gate electrode of thetransistor 1000 and therefore has a function similar to that of afloating gate of a floating gate transistor used for a nonvolatilememory element. Therefore, in drawings, a portion where the drainelectrode (or the source electrode) of the transistor 1010 iselectrically connected to the gate electrode of the transistor 1000 iscalled a floating gate portion FG in some cases. When the transistor1010 is off, the floating gate portion FG can be regarded as beingembedded in an insulator and thus charge is held in the floating gateportion FG. The off-state current of the transistor 1010 including anoxide semiconductor is smaller than or equal to 1/100000 of theoff-state current of a transistor including a silicon semiconductor orthe like; thus, loss of the charge accumulated in the floating gateportion FG due to leakage of the transistor 1010 is negligible. That is,with the transistor 1010 including an oxide semiconductor, a nonvolatilememory device which can hold data without being supplied with power canbe realized.

For example, when the off-state current of the transistor 1010 at roomtemperature is 10 zA (1 zA (zeptoampere) is 1×10⁻²¹ A) or less and thecapacitance of the capacitor 1020 is approximately 10 fF, data can beheld for 10⁴ seconds or longer. It is needless to say that the holdingtime depends on transistor characteristics and capacitance.

Further, in that case, the problem of deterioration of a gate insulatingfilm (tunnel insulating film), which has been a problem of aconventional floating gate transistor, does not exist. That is, theproblem of deterioration of a gate insulating film due to injection ofelectrons into a floating gate, which is a conventional problem, can besolved. This means that there is no limit on the number of writing timesin principle. Furthermore, a high voltage needed for writing or erasingin a conventional floating gate transistor is not necessary.

Components such as transistors in the semiconductor device in FIG. 6A1can be regarded as including resistors and capacitors as illustrated inFIG. 6A2. That is, in FIG. 6A2, the transistor 1000 and the capacitor1020 are each regarded as including a resistor and a capacitor. R1 andC1 denote the resistance and the capacitance of the capacitor 1020,respectively. The resistance R1 corresponds to the resistance of theinsulating film included in the capacitor 1020. R2 and C2 denote theresistance and the capacitance of the transistor 1000, respectively. Theresistance R2 corresponds to the resistance of the gate insulating filmat the time when the transistor 1000 is on. The capacitance C2corresponds to a so-called gate capacitance (capacitance formed betweenthe gate electrode and the source or drain electrode, and capacitanceformed between the gate electrode and the channel formation region).

The value of resistance (also referred to as effective resistance)between the source electrode and the drain electrode in the case wherethe transistor 1010 is off is denoted by ROS. When the resistance R1 andthe resistance R2 satisfy the following relations of R1 ROS (R1 ishigher than or equal to ROS) and R2 ROS (R2 is higher than or equal toROS) under the condition that gate leakage of the transistor 1010 issufficiently small, a period for holding charge (also referred to as adata retention period) is determined mainly by the off-state current ofthe transistor 1010.

On the other hand, when the above relations are not satisfied, it isdifficult to secure a sufficient retention period even if the off-statecurrent of the transistor 1010 is sufficiently small. This is because aleakage current other than the off-state current of the transistor 1010(e.g., a leakage current generated between the source electrode and thegate electrode) is large. Thus, it is preferable that the semiconductordevice disclosed in this embodiment satisfy the above relations.

Moreover, the capacitance C1 and the capacitance C2 preferably satisfythe relation: C1≧C2 (C1 is larger than or equal to C2). This is becauseif the capacitance C1 is large, the potential of the fifth wiring can besupplied to the floating gate portion FG efficiently at the time ofcontrolling the potential of the floating gate portion FG by the fifthwiring, and a difference between potentials (e.g., the reading potentialand a non-reading potential) supplied to the fifth wiring can be madesmall.

When the above relation is satisfied, a more favorable semiconductordevice can be realized. Note that the resistance R1 and the resistanceR2 are controlled by the gate insulating film of the transistor 1000 andthe insulating film of the capacitor 1020. The same can be said to thecapacitance C1 and the capacitance C2. Therefore, it is preferable thatthe material, the thickness, and the like of the gate insulating film beset as appropriate to satisfy the above relations.

In the semiconductor device of this embodiment, the floating gateportion FG has a function equivalent to that of a floating gate of afloating gate transistor in a flash memory or the like, but the floatinggate portion FG of this embodiment has a feature essentially differentfrom that of the floating gate of the flash memory or the like. In aflash memory, since a voltage applied to a control gate is high, it isnecessary to keep a proper distance between cells in order to preventthe potential from adversely affecting a floating gate of the adjacentcell. This is one factor inhibiting higher integration of semiconductordevices. The factor is attributed to the following basic principle of aflash memory: a tunneling current is generated by application of a highelectric field.

Further, because of the above principle of a flash memory, deteriorationof an insulating film proceeds and thus another problem of the limit onthe number of rewriting times (approximately more than or equal to 10⁴times and less than or equal to 10⁵ times) arises.

The semiconductor device according to the invention disclosed herein isoperated by switching of a transistor including an oxide semiconductorand does not use the above-described principle of charge injection by atunneling current. That is, unlike a flash memory, a high electric fieldfor injection of charge is not necessary. Accordingly, it is notnecessary to consider an influence of a high electric field from acontrol gate on an adjacent cell, which facilitates high integration.

Further, since the principle of charge injection by a tunneling currentis not used, it can be said that there is no cause for deterioration ofa memory cell. In other words, the semiconductor device according to theinvention disclosed herein has higher durability and reliability than aflash memory.

In addition, the semiconductor device according to the invention hasadvantages over a flash memory in that a high electric field isunnecessary and a large peripheral circuit (such as a booster circuit)is unnecessary.

In the case where the relative permittivity εr1 of the insulating filmincluded in the capacitor 1020 is different from the relativepermittivity εr2 of the insulating film included in the transistor 1000,it is easy to satisfy C1≧C2 (C1 is larger than or equal to C2) while2·S2≧S1 (2·S2 is larger than or equal to S1), or preferably S2≧S1 (S2 islarger than or equal to S1) is satisfied, where S1 is the area of theinsulating film included in the capacitor 1020 and S2 is the area of theinsulating film forming the gate capacitance of the transistor 1000.That is, it is easy to satisfy C1≧C2 while it is satisfied that the areaof the insulating film included in the capacitor 1020 is small.Specifically, for example, a film formed of a high-k material such ashafnium oxide or a stack including a film formed of a high-k materialsuch as hafnium oxide and a film formed of an oxide semiconductor isused for the insulating film included in the capacitor 1020 so that εr1can be set to 10 or more, preferably 15 or more, and silicon oxide isused for the insulating film forming the gate capacitance so that εr2can be set to higher than or equal to 3 and lower than or equal to 4.

A combination of such structures enables higher integration of thesemiconductor device according to the invention disclosed herein.

Note that an n-channel transistor in which electrons are majoritycarriers is used in the above description; it is needless to say that ap-channel transistor in which holes are majority carriers can be usedinstead of the n-channel transistor.

As described above, the semiconductor device according to an embodimentof the invention disclosed herein has a non-volatile memory cell thatincludes a writing transistor in which a leakage current between asource and a drain in an off state (an off-state current) is small, areading transistor including a semiconductor material different fromthat of the writing transistor, and a capacitor.

With a normal silicon semiconductor, it is difficult to decrease theleakage current (the off-state current) to approximately 100 zA (1×10⁻¹⁹A) or less at ambient temperature (e.g., 25° C.), whereas this can beachieved with a transistor including an oxide semiconductor which isprocessed under an appropriate condition. Therefore, a transistorincluding an oxide semiconductor is preferably used as the writingtransistor.

In addition, a transistor including an oxide semiconductor has a smallsubthreshold swing (S value), so that the switching rate can besufficiently high even if mobility is comparatively low. Therefore, byusing the transistor as the writing transistor, the rise of a writingpulse given to the floating gate portion FG can be very sharp. Further,since the off-state current is small, the amount of charge held in thefloating gate portion FG can be reduced. That is, by using a transistorincluding an oxide semiconductor as the writing transistor, rewriting ofdata can be performed at high speed.

Although there is no limitation on the off-state current of the readingtransistor, a transistor that operates at high speed is preferably usedas the reading transistor in order to increase the readout speed. Forexample, a transistor having a switching rate of 1 nanosecond or loweris preferably used as the reading transistor.

In this manner, when a transistor including an oxide semiconductor isused as a writing transistor, and a transistor including a semiconductormaterial other than an oxide semiconductor is used as a readingtransistor, a semiconductor device capable of holding data for a longtime and reading data at high speed, which can be used as a memorydevice, can be obtained.

Furthermore, with the use of the transistor described in the aboveembodiment as a writing transistor, a short channel effect of thewriting transistor can be suppressed, and miniaturization can beachieved. Accordingly, higher integration of a semiconductor devicewhich can be used as a memory device can be achieved.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 4

In this embodiment, an example of application of a semiconductor deviceaccording to one embodiment of the invention disclosed herein will bedescribed with reference to FIGS. 7A and 7B and FIGS. 8A to 8C. Here, anexample of a memory device will be described. Note that in some circuitdiagrams, “OS” is written beside a transistor in order to indicate thatthe transistor includes an oxide semiconductor.

FIGS. 7A and 7B are circuit diagrams of semiconductor devices, which canbe used as memory devices, each including a plurality of semiconductordevices (hereinafter also referred to as memory cells 1050) illustratedin FIG. 6A1. FIG. 7A is a circuit diagram of a so-called NANDsemiconductor device in which the memory cells 1050 are connected inseries, and FIG. 7B is a circuit diagram of a so-called NORsemiconductor device in which the memory cells 1050 are connected inparallel.

The semiconductor device in FIG. 7A includes a source line SL, a bitline BL, a first signal line 51, a plurality of second signal lines S2,a plurality of word lines WL, and a plurality of memory cells 1050. InFIG. 7A, one source line SL and one bit line BL are provided in thesemiconductor device; however, an embodiment of the invention disclosedherein is not limited to this structure. A plurality of source lines SLand a plurality of bit lines BL may be provided.

In each of the memory cells 1050, the gate electrode of the transistor1000, the drain electrode (or the source electrode) of the transistor1010, and one electrode of the capacitor 1020 are electrically connectedto one another. The first signal line S1 and the source electrode (orthe drain electrode) of the transistor 1010 are electrically connectedto each other, and the second signal line S2 and the gate electrode ofthe transistor 1010 are electrically connected to each other. The wordline WL and the other electrode of the capacitor 1020 are electricallyconnected to each other.

Further, the source electrode of the transistor 1000 included in thememory cell 1050 is electrically connected to the drain electrode of thetransistor 1000 in the adjacent memory cell 1050. The drain electrode ofthe transistor 1000 included in the memory cell 1050 is electricallyconnected to the source electrode of the transistor 1000 in the adjacentmemory cell 1050. Note that the drain electrode of the transistor 1000included in the memory cell 1050 at one end of the plurality of memorycells connected in series is electrically connected to the bit line BL.The source electrode of the transistor 1000 included in the memory cell1050 at the other end of the plurality of memory cells connected inseries is electrically connected to the source line SL.

In the semiconductor device in FIG. 7A, writing operation and readingoperation are performed for each row. The writing operation is performedas follows. A potential at which the transistor 1010 is on is suppliedto the second signal line S2 of a row where writing is to be performed,so that the transistor 1010 of the row where writing is to be performedis turned on. Accordingly, a potential of the first signal line S1 issupplied to the gate electrode of the transistor 1000 of the specifiedrow, so that predetermined charge is given to the gate electrode. Thus,data can be written to the memory cell of the specified row.

Further, the reading operation is performed as follows. First, apotential at which the transistor 1000 is on regardless of charge of thegate electrode of the transistor 1000 is supplied to the word lines WLof the rows other than the row where reading is to be performed, so thatthe transistors 1000 of the rows other than the row where reading is tobe performed are turned on. Then, a potential (reading potential) atwhich an on state or an off state of the transistor 1000 is determineddepending on charge of the gate electrode of the transistor 1000 issupplied to the word line WL of the row where reading is to beperformed. After that, a constant potential is supplied to the sourceline SL so that a reading circuit (not illustrated) connected to the bitline BL is operated. Here, the plurality of transistors 1000 between thesource line SL and the bit line BL are turned on except the transistor1000 of the row where reading is to be performed; therefore, conductancebetween the source line SL and the bit line BL is determined by thestate of the transistor 1000 (whether on or off) of the row wherereading is to be performed. Since the conductance of the transistorvaries depending on the charge in the gate electrode of the transistor1000 of the row where reading is to be performed, a potential of the bitline BL also varies accordingly. By reading the potential of the bitline BL with the reading circuit, data can be read from the memory cellof the specified row.

The semiconductor device in FIG. 7B includes a plurality of source linesSL, a plurality of bit lines BL, a plurality of first signal lines 51, aplurality of second signal lines S2, a plurality of word lines WL, and aplurality of memory cells 1050. A gate electrode of the transistor 1000,the drain electrode (or the source electrode) of the transistor 1010,and one electrode of the capacitor 1020 are electrically connected toone another. The source line SL and the source electrode of thetransistor 1000 are electrically connected to each other. The bit lineBL and the drain electrode of the transistor 1000 are electricallyconnected to each other. The first signal line 51 and the sourceelectrode (or the drain electrode) of the transistor 1010 areelectrically connected to each other, and the second signal line S2 andthe gate electrode of the transistor 1010 are electrically connected toeach other. The word line WL and the other electrode of the capacitor1020 are electrically connected to each other.

In the semiconductor device in FIG. 7B, writing operation and readingoperation are performed per row. The writing operation is performed in amanner similar to that of the semiconductor device in FIG. 7A. Thereading operation is performed as follows. First, a potential at whichthe transistor 1000 is off regardless of charge of the gate electrode ofthe transistor 1000 is supplied to the word lines WL of the rows otherthan the row where reading is to be performed, so that the transistors1000 of the rows other than the row where reading is to be performed areturned off. Then, a potential (reading potential) at which an on stateor an off state of the transistor 1000 is determined depending on chargeof the gate electrode of the transistor 1000 is supplied to the wordline WL of the row where reading is to be performed. After that, aconstant potential is supplied to the source line SL so that a readingcircuit (not illustrated) connected to the bit line BL is operated.Here, conductance between the source line SL and the bit line BL isdetermined by the state of the transistor 1000 (whether on or off) ofthe row where reading is to be performed. That is, a potential of thebit line BL depends on charge of the gate electrode of the transistor1000 of the row where reading is to be performed. By reading thepotential of the bit line BL with the reading circuit, data can be readfrom the memory cell of the specified row.

Although the amount of data which can be stored in each of the memorycells 1050 is one bit in the above description, the structure of thesemiconductor device of this embodiment is not limited to this example.The amount of data which is held in each of the memory cells 1050 may beincreased by preparing three or more kinds of potentials to be suppliedto the gate electrode of the transistor 1000. For example, in the casewhere four kinds of potentials are supplied to the gate electrode of thetransistor 1000, data of two bits can be held in each of the memorycells.

Next, an example of a reading circuit which can be used for thesemiconductor devices illustrated in FIGS. 7A and 7B and the like willbe described with reference to FIGS. 8A to 8C.

FIG. 8A illustrates an outline of the reading circuit. The readingcircuit includes a transistor and a sense amplifier circuit.

At the time of reading data, a terminal A is connected to a bit line BLto which a memory cell from which data is to be read is connected.Further, a bias potential Vbias is applied to a gate electrode of thetransistor so that a potential of the terminal A is controlled.

The resistance of the memory cell 1050 varies depending on stored data.Specifically, when the transistor 1000 of the selected memory cell 1050is on, the memory cell 1050 has a low resistance, whereas when thetransistor 1000 of the selected memory cell 1050 is off, the memory cell1050 has a high resistance.

When the memory cell has a high resistance, the potential of theterminal A is higher than a reference potential Vref and the senseamplifier circuit outputs a potential corresponding to the potential ofthe terminal A. On the other hand, when the memory cell has a lowresistance, the potential of the terminal A is lower than the referencepotential Vref and the sense amplifier circuit outputs a potentialcorresponding to the potential of the terminal A.

In this manner, by using the reading circuit, data can be read from thememory cell. Note that the reading circuit of this embodiment is oneexample. Another circuit may be used. The reading circuit may furtherinclude a precharge circuit. Instead of the reference potential Vref, areference bit line may be connected to the sense amplifier circuit.

FIG. 8B illustrates a differential sense amplifier which is an exampleof sense amplifier circuits. The differential sense amplifier has inputterminals Vin(+) and Vin(−) and an output terminal Vout, and amplifies adifference between potentials of the input terminals Vin(+) and Vin(−).If Vin(+)>Vin(−), the output from Vout is relatively high, whereas ifVin(+)<Vin(−), the output from Vout is relatively low. In the case wherethe differential sense amplifier is used for the reading circuit, one ofthe input terminals Vin(+) and Vin(−) is connected to the terminal A,and the reference potential Vref is supplied to the other of the inputterminals Vin(+) and Vin(−).

FIG. 8C illustrates a latch sense amplifier which is an example of senseamplifier circuits. The latch sense amplifier has input/output terminalsV1 and V2 and input terminals for control signals Sp and Sn. First, thecontrol signal Sp is set high and the control signal Sn is set low, anda power supply potential Vdd is interrupted. Then, potentials V1 in andV2 in to be compared are supplied to the input/output terminals V1 andV2. After that, the control signal Sp is set low and the control signalSn is set high, and the power supply potential Vdd is supplied. If thepotentials V1 in and V2 in to be compared satisfy V1 in>V2 in, theoutput from the input/output terminal V1 is high and the output from theinput/output terminal V2 is low, whereas if the potentials satisfy V1in<V2 in, the output from the input/output terminal V1 is low and theoutput from the input/output terminal V2 is high. By utilizing such arelationship, the difference between potentials V1 in and V2 in can beamplified. In the case where the latch sense amplifier is used for thereading circuit, one of the input/output terminals V1 and V2 isconnected to the terminal A and an output terminal through a switch, andthe reference potential Vref is supplied to the other of theinput/output terminals V1 and V2.

With the use of the transistor described in the above embodiment as awriting transistor of a memory cell in the above-described semiconductordevice which can be used as a memory device, a short channel effect ofthe writing transistor can be suppressed, and miniaturization can beachieved. Accordingly, higher integration of a semiconductor devicewhich can be used as a memory device can be achieved.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 5

In this embodiment, an example of application of a semiconductor deviceaccording to one embodiment of the invention disclosed herein will bedescribed with reference to FIG. 9. Here, a central processing unit(CPU) will be described.

FIG. 9 illustrates an example of a block diagram of a CPU. A CPU 1101illustrated in FIG. 9 includes a timing control circuit 1102, aninstruction decoder 1103, a register array 1104, an address logic andbuffer circuit 1105, a data bus interface 1106, an arithmetic logic unit(ALU) 1107, an instruction register 1108, and the like.

These circuits are manufactured using the transistor described in theabove embodiment, an inverter circuit, a resistor, a capacitor, and thelike. Because the transistor described in the above embodiment canachieve an extremely small off-state current, a reduction in powerconsumption of the CPU 1101 can be realized. Furthermore, with the useof the transistor described in the above embodiment, a short channeleffect of the transistor can be suppressed, and miniaturization can beachieved.

Circuits included in the CPU 1101 will be briefly described below. Thetiming control circuit 1102 receives instructions from the outside,converts the instructions into information for the inside, and transmitsthe information to another block. In addition, the timing controlcircuit 1102 gives directions such as reading and writing of memory datato the outside, according to internal operation. The instruction decoder1103 functions to convert instructions from the outside intoinstructions for the inside. The register array 1104 functions totemporarily store data. The address logic and buffer circuit 1105functions to specify the address of an external memory. The data businterface 1106 functions to take data in and out of an external memoryor a device such as a printer. The ALU 1107 functions to perform anoperation. The instruction register 1108 functions to temporarily storeinstructions. The CPU includes such a combination of circuits.

With the use of the transistor described in the above embodiment in atleast a part of the CPU 1101, a short channel effect of the transistorcan be suppressed, and miniaturization can be achieved. Thus, higherintegration of the CPU 1101 can be achieved.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 6

In this embodiment, an example of application of a semiconductor deviceaccording to one embodiment of the invention disclosed herein will bedescribed with reference to FIGS. 10A and 10B. Here, an example of asemiconductor device having an image sensor function for readinginformation of an object will be described. Note that in some circuitdiagrams, “OS” is written beside a transistor in order to indicate thatthe transistor includes an oxide semiconductor.

FIG. 10A illustrates an example of a semiconductor device having animage sensor function. FIG. 10A is an equivalent circuit diagram of aphotosensor, and FIG. 10B is a cross-sectional view of a part of thephotosensor.

One electrode of a photodiode 1202 is electrically connected to aphotodiode reset signal line 1212, and the other electrode of thephotodiode 1202 is electrically connected to a gate of a transistor1204. One of a source electrode and a drain electrode of the transistor1204 is electrically connected to a photosensor reference signal line1218, and the other of the source electrode and the drain electrode ofthe transistor 1204 is electrically connected to one of a sourceelectrode and a drain electrode of a transistor 1206. A gate electrodeof the transistor 1206 is electrically connected to a gate signal line1214, and the other of the source electrode and the drain electrode ofthe transistor 1206 is electrically connected to a photosensor outputsignal line 1216.

Here, transistors including an oxide semiconductor are used as thetransistor 1204 and the transistor 1206 illustrated in FIG. 10A. Here,the transistor described in the above embodiment can be used as thetransistors including an oxide semiconductor. Because the transistordescribed in the above embodiment can achieve an extremely small leakagecurrent in an off state, the photodetection accuracy of the photosensorcan be improved. Furthermore, with the use of the transistor describedin the above embodiment, a short channel effect of the transistor can besuppressed, and miniaturization can be achieved. Thus, the area of thephotodiode can be increased, and the photodetection accuracy of thephotosensor can be improved.

FIG. 10B is a cross-sectional view illustrating the photodiode 1202 andthe transistor 1204 in the photosensor. The photodiode 1202 functioningas a sensor and the transistor 1204 are provided over a substrate 1222having an insulating surface (a TFT substrate). A substrate 1224 isprovided over the photodiode 1202 and the transistor 1204 using anadhesive layer 1228. In addition, an insulating film 1234, an interlayerinsulating film 1236, and an interlayer insulating film 1238 areprovided over the transistor 1204.

In addition, a gate electrode layer 1240 is provided in the same layeras the gate electrode of the transistor 1204 so as to be electricallyconnected to the gate electrode. The gate electrode layer 1240 iselectrically connected to an electrode layer 1242 provided over theinterlayer insulating film 1236, at an opening formed in the insulatingfilm 1234 and the interlayer insulating film 1236. Because thephotodiode 1202 is formed over the electrode layer 1242, the photodiode1202 and the transistor 1204 are electrically connected to each otherthrough the gate electrode layer 1240 and the electrode layer 1242.

The photodiode 1202 has a structure in which a first semiconductor film1226 a, a second semiconductor film 1226 b, and a third semiconductorfilm 1226 c are stacked in this order over the electrode layer 1242. Inother words, the first semiconductor film 1226 a of the photodiode 1202is electrically connected to the electrode layer 1242. In addition, thethird semiconductor film 1226 c of the photodiode 1202 is electricallyconnected to an electrode layer 1244 provided over the interlayerinsulating film 1238.

Here, a PIN photodiode is given as an example, in which a semiconductorfilm having n-type conductivity as the first semiconductor film 1226 a,a high-resistance semiconductor film (an i-type semiconductor film) asthe second semiconductor film 1226 b, and a semiconductor film havingp-type conductivity as the third semiconductor film 1226 c are stacked.

The first semiconductor film 1226 a is an n-type semiconductor film andis formed from an amorphous silicon film containing an impurity elementimparting n-type conductivity. In the formation of the firstsemiconductor film 1226 a, a plasma CVD method with the use of asemiconductor source gas containing an impurity element belonging toGroup 15 (e.g., phosphorus (P)) is employed. As the semiconductor sourcegas, silane (SiH₄) may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃,SiCl₄, SiF₄, or the like may be used. Alternatively, an amorphoussilicon film which does not contain an impurity element may be formed,and then, an impurity element may be introduced into the amorphoussilicon film by a diffusion method or an ion implantation method. Afterthe impurity element is introduced by an ion implantation method or thelike, heating or the like may be conducted in order to diffuse theimpurity element. In this case, as a method for forming the amorphoussilicon film, an LPCVD method, a vapor deposition method, a sputteringmethod, or the like may be used. The first semiconductor film 1226 a ispreferably formed so as to have a thickness of greater than or equal to20 nm and less than or equal to 200 nm.

The second semiconductor film 1226 b is an i-type semiconductor film (anintrinsic semiconductor film) and is formed from an amorphous siliconfilm. In the formation of the second semiconductor film 1226 b, theamorphous silicon film is formed by a plasma CVD method with use of asemiconductor source gas. As the semiconductor source gas, silane (SiH₄)may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or thelike may be used. The second semiconductor film 1226 b may alternativelybe formed by an LPCVD method, a vapor deposition method, a sputteringmethod, or the like. The second semiconductor film 1226 b is preferablyformed so as to have a thickness of greater than or equal to 200 nm andless than or equal to 1000 nm.

The third semiconductor film 1226 c is a p-type semiconductor film andis formed from an amorphous silicon film containing an impurity elementimparting p-type conductivity. In the formation of the thirdsemiconductor film 1226 c, a plasma CVD method with the use of asemiconductor source gas containing an impurity element belonging toGroup 13 (e.g., boron (B)) is employed. As the semiconductor source gas,silane (SiH₄) may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄,SiF₄, or the like may be used. Alternatively, an amorphous silicon filmwhich does not contain an impurity element may be formed, and then, animpurity element may be introduced into the amorphous silicon film by adiffusion method or an ion implantation method. After the impurityelement is introduced by an ion implantation method or the like, heatingor the like may be performed in order to diffuse the impurity element.In this case, as a method for forming the amorphous silicon film, anLPCVD method, a vapor deposition method, a sputtering method, or thelike may be used. The third semiconductor film 1226 c is preferablyformed so as to have a thickness of greater than or equal to 10 nm andless than or equal to 50 nm

The first semiconductor film 1226 a, the second semiconductor film 1226b, and the third semiconductor film 1226 c are not necessarily formedusing an amorphous semiconductor, and they may be formed using apolycrystalline semiconductor or a microcrystalline semiconductor (or asemi-amorphous semiconductor (SAS)).

The microcrystalline semiconductor belongs to a metastable state whichis an intermediate state between an amorphous state and a single crystalstate according to Gibbs free energy. That is, the microcrystallinesemiconductor is a semiconductor having a third state which is stable interms of free energy and has a short range order and lattice distortion.In the microcrystalline semiconductor, columnar or needle-like crystalsgrow in a normal direction with respect to a surface of a substrate. TheRaman spectrum of microcrystalline silicon, which is a typical exampleof the microcrystalline semiconductor, is shifted to a smallerwavenumber region than 520 cm⁻¹ which represents single crystal silicon.That is, the peak of the Raman spectrum of microcrystalline siliconexists between 520 cm⁻¹ which represents single crystal silicon and 480cm⁻¹ which represents amorphous silicon. The microcrystallinesemiconductor includes at least 1 at. % of hydrogen or halogen toterminate a dangling bond. Moreover, a rare gas element such as helium,argon, krypton, or neon may be included to further promote latticedistortion, so that a favorable microcrystalline semiconductor film withenhanced stability can be obtained.

This microcrystalline semiconductor film can be formed by ahigh-frequency plasma CVD method with a frequency of higher than orequal to several tens of megahertz and lower than or equal to severalhundreds of megahertz or a microwave plasma CVD method with a frequencyof 1 GHz or higher. Typically, the microcrystalline semiconductor filmcan be formed using a gas obtained by diluting a silicon hydride, suchas SiH₄, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, or SiF₄, with hydrogenAlternatively, the microcrystalline semiconductor film can be formedusing a gas including a silicon hydride and hydrogen which is dilutedwith one or more rare gas elements selected from helium, argon, krypton,and neon. In this case, the flow rate of hydrogen is set 5 times to 200times, preferably 50 times to 150 times, further preferably 100 times,as high as that of a silicon hydride. Furthermore, a gas includingsilicon may be mixed with a carbide gas such as CH₄ or C₂H₆, a germaniumgas such as GeH₄ or GeF₄, F₂, or the like.

In addition, the mobility of holes generated by the photoelectric effectis lower than the mobility of electrons. Therefore, a PIN photodiode hasbetter characteristics when a surface on the p-type semiconductor filmside is used as a light-receiving plane. Here, an example where thephotodiode 1202 receives incident light 1230 from the substrate 1224side and converts it into electric signals is described. Further, lightfrom a side on which the semiconductor film having a conductivity typeopposite to that of the semiconductor film on the light-receiving planeside is disturbance light; therefore, the electrode layer 1242 ispreferably formed using a light-blocking conductive film. Note that then-type semiconductor film side may alternatively be a light-receivingplane.

In addition, when the incident light 1230 enters from the substrate 1224side, the oxide semiconductor film of the transistor 1204 can beshielded from the incident light 1230 by the gate electrode of thetransistor 1204.

The insulating film 1234, the interlayer insulating film 1236, and theinterlayer insulating film 1238 can be formed using an insulatingmaterial by a method such as a sputtering method, an SOG method, a spincoating method, a dipping method, a spray coating method, or a dropletdischarge method (e.g., an ink-jet method, screen printing, or offsetprinting), or with a tool such as a doctor knife, a roll coater, acurtain coater, or a knife coater, depending on the material.

The insulating film 1234 may be a single layer or stacked layers of aninorganic insulating material, using any of oxide insulating films ornitride insulating films such as a silicon oxide layer, a siliconoxynitride layer, a silicon nitride layer, a silicon nitride oxidelayer, an aluminum oxide layer, an aluminum oxynitride layer, analuminum nitride layer, or an aluminum nitride oxide layer. In addition,a high-quality insulating film which is dense and has high withstandvoltage can be formed by a high-density plasma CVD method usingmicrowaves (2.45 GHz), which is preferable.

For a reduction of surface roughness, an insulating film functioning asa planarization insulating film is preferably used as the interlayerinsulating films 1236 and 1238. For example, the interlayer insulatingfilms 1236 and 1238 can be formed using an organic insulating materialhaving heat resistance such as polyimide, acrylic, benzocyclobutene,polyamide, or epoxy. Other than such organic insulating materials, it ispossible to use a single layer or stacked layers of a low dielectricconstant material (a low-k material), a siloxane-based resin,phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or thelike.

The photodiode 1202 can read information of an object by detecting theincident light 1230. Note that a light source such as a backlight can beused at the time of reading information of an object.

In the photosensor described above, the transistor described in theabove embodiment can be used as the transistor including an oxidesemiconductor. Because the transistor described in the above embodimentcan achieve an extremely small leakage current in an off state, thephotodetection accuracy of the photosensor can be improved. Furthermore,with the use of the transistor described in the above embodiment, ashort channel effect of the transistor can be suppressed, andminiaturization can be achieved. Thus, the area of the photodiode can beincreased, and the photodetection accuracy of the photosensor can beimproved.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 7

In this embodiment, the cases where any of the semiconductor devicesdescribed in the above embodiments is applied to electronic devices willbe described with reference to FIGS. 11A to 11F. The cases where any ofthe above-described semiconductor devices is applied to electronicdevices such as a computer, a mobile phone set (also referred to as amobile phone or a mobile phone device), a portable information terminal(including a portable game machine, an audio reproducing device, and thelike), a digital camera, a digital video camera, electronic paper, atelevision set (also referred to as a television or a televisionreceiver), and the like will be described in this embodiment.

FIG. 11A illustrates a laptop personal computer, which includes ahousing 701, a housing 702, a display portion 703, a keyboard 704, andthe like. At least one of the housings 701 and 702 is provided with anyof the semiconductor devices described in the above embodiments. Thus, alaptop personal computer which operates at high speed and consumes lowpower can be realized for example.

FIG. 11B illustrates a portable information terminal (PDA). A main body711 is provided with a display portion 713, an external interface 715,operation buttons 714, and the like. Further, a stylus 712 for operationof the portable information terminal, or the like is provided. The mainbody 711 is provided with any of the semiconductor devices described inthe above embodiments. Thus, a portable information terminal whichoperates at high speed and consumes low power can be realized forexample.

FIG. 11C illustrates an electronic book reader 720 incorporatingelectronic paper, which includes two housings, a housing 721 and ahousing 723. The housing 721 and the housing 723 include a displayportion 725 and a display portion 727, respectively. The housing 721 isconnected to the housing 723 by a hinge 737, so that the electronic bookreader 720 can be opened and closed using the hinge 737 as an axis. Inaddition, the housing 721 is provided with a power switch 731, operationkeys 733, a speaker 735, and the like. At least one of the housings 721and 723 is provided with any of the semiconductor devices described inthe above embodiments. Thus, an electronic book reader which operates athigh speed and consumes low power can be realized for example.

FIG. 11D illustrates a mobile phone set, which includes two housings, ahousing 740 and a housing 741. Moreover, the housings 740 and 741 in astate where they are developed as illustrated in FIG. 11D can be slid sothat one is lapped over the other. Therefore, the size of the mobilephone set can be reduced, which makes the mobile phone set suitable forbeing carried around. The housing 741 includes a display panel 742, aspeaker 743, a microphone 744, operation keys 745, a pointing device746, a camera lens 747, an external connection terminal 748, and thelike. The housing 740 includes a solar cell 749 for charging the mobilephone set, an external memory slot 750, and the like. An antenna isincorporated in the housing 741. At least one of the housings 740 and741 is provided with any of the semiconductor devices described in theabove embodiments. Thus, a mobile phone set which operates at high speedand consumes low power can be realized for example.

FIG. 11E illustrates a digital camera, which includes a main body 761, adisplay portion 767, an eyepiece 763, an operation switch 764, a displayportion 765, a battery 766, and the like. The main body 761 is providedwith any of the semiconductor devices described in the aboveembodiments. Thus, a digital camera which operates at high speed andconsumes low power can be realized for example.

FIG. 11F illustrates a television set 770, which includes a housing 771,a display portion 773, a stand 775, and the like. The television set 770can be operated with a switch included in the housing 771 or with aremote controller 780. The housing 771 and the remote controller 780 areprovided with any of the semiconductor devices described in the aboveembodiments. Thus, a television set which operates at high speed andconsumes low power can be realized for example.

As described above, the electronic devices described in this embodimenteach include any of the semiconductor devices according to the aboveembodiments. Thus, electronic devices whose high-speed operation and lowpower consumption can be realized by miniaturization of thesemiconductor device can be obtained.

EXAMPLE 1

In this example, results of verification, by computational simulation,of the effect of the invention disclosed herein in terms of suppressionof a short channel effect will be described. Note that calculations (afirst calculation and a second calculation) were carried out using adevice simulator “Atlas” produced by Silvaco Data Systems Inc.

The first calculation was carried out using models illustrated in FIGS.12A and 12B. FIG. 12A illustrates a transistor having a structure inwhich there is a small difference in height between an upper surface ofan insulating film and an upper surface of a source electrode or a drainelectrode (hereinafter referred to as Structure A), according to oneembodiment of the invention disclosed herein. FIG. 12B illustrates atransistor having a structure without such a difference in height(hereinafter referred to as Structure B), as a comparative example. Notethat although Structure A employs an insulating film 143 a asillustrated in FIG. 12A for simplicity of the calculation, there is notan essential difference between Structure A and the structureillustrated in FIG. 1A or the like.

In the calculation, the inclination angle θ of a side surface of aregion where the difference in height is made (hereinafter referred toas a projecting region) and the height h of the projecting region werevariables. Note that components denoted by reference numerals in FIGS.12A and 12B correspond to the components denoted by the same referencenumerals in the above embodiment. In addition, the calculation models ofthis example do not include some components such as the insulating film150, but this does not affect the result of calculation.

Other parameters used for the calculation are as follows.

Thickness of oxide semiconductor film: 10 nm

Material of oxide semiconductor film: In—Ga—Zn—O-based metal oxide (bandgap E_(g): 3.15 eV, electron affinity (χ): 4.3 eV, relativepermittivity: 15, electron mobility: 10 cm²/Vs)

Thickness of gate insulating film: 10 nm

Material of gate insulating film: hafnium oxide (relative permittivity:15)

Material of source and drain electrodes: titanium nitride (workfunction: 3.9 eV)

Material of gate electrode: tungsten (work function: 4.9 eV)

The results of the first calculation are shown in FIG. 13, FIG. 14, andFIG. 15. FIG. 13 shows the relation between gate voltage V_(G) (V) anddrain current I_(D) (A); FIG. 14, the relation between channel length L(nm) and threshold voltage V_(th) (V); and FIG. 15, the relation betweenchannel length L (nm) and S value (V/dec). Note that each of FIG. 13,FIG. 14, and FIG. 15 shows results obtained with different inclinationangles 0 of 45°, 60° , and 90° and heights h of 5 nm, 10 nm, and 20 nm.

The results in FIG. 13, FIG. 14, and FIG. 15 show that there is nosignificant difference between Structure A and Structure B when thechannel length L is more than 100 nm, whereas a negative shift inthreshold voltage V_(th) and an increase in S value are suppressed inStructure A when the channel length L is 100 nm or less. That is, ashort channel effect can be suppressed in Structure A, in comparisonwith Structure B.

The second calculation was carried out using models illustrated in FIGS.16A and 16B. FIG. 16A illustrates Structure A, and FIG. 16B illustratesStructure B. A difference between the first and second calculations isthe way of defining the channel length L: in the first calculation, thedistance between the source electrode and the drain electrode is definedas the channel length L, whereas the channel length L in the secondcalculation is measured along the projecting shape of the insulatingfilm. In other words, in FIG. 16A, the channel length L equals L_(s)plus L_(c) plus L_(d). By defining the channel length L in FIG. 16A inthis manner, the effect of an increase in effective value of the channellength L can be counteracted, and an effect purely resulting from theshape can be observed.

FIG. 17 shows the relation between gate voltage V_(G) (V) and draincurrent I_(D) (A) which is obtained from the results of the secondcalculation. Here, the inclination angle θ is fixed to 90° , and theheight h is set to 5 nm, 10 nm, and 20 nm FIG. 17 shows that a negativeshift in threshold voltage V_(th) is suppressed in the structure inwhich there is a small difference in height between the upper surface ofthe insulating film and the upper surface of the source or drainelectrode (Structure A), owing to its shape. In other words, it can beunderstood that the shape suppresses a short channel effect.

Note that it can be seen from the results of the first and secondcalculations that a short channel effect is less easily caused as theheight h increases. However, if there is a large difference in height, adecrease in coverage may cause a disconnection of the oxidesemiconductor film or the like. Therefore, the difference in heightshould be set to 30 nm or less, preferably 20 nm or less.

EXAMPLE 2

In this example, detailed results of off-state current I_(off) of atransistor including an intrinsic oxide semiconductor will be described.Note that Sentaurus Device (TCAD software produced by Synopsys, Inc.)was used for calculation. In addition, the Shockley-Read-Hall (SRH)model and the Auger recombination model were used as carrierrecombination models.

The structure of a transistor used for the calculation is the oneillustrated in FIG. 18. The transistor illustrated in FIG. 18 includesan insulating film 143 a (material: silicon oxide), a source electrode142 c and a drain electrode 142 d (material: titanium nitride), an oxidesemiconductor film 144 (material: In—Ga—Zn—O-based oxide semiconductor,thickness: 10 nm) in contact with a surface of the insulating film 143a, a surface of the source electrode 142 c, and a surface of the drainelectrode 142 d, a gate insulating film 146 (material: hafnium oxide,thickness: 10 nm) covering the oxide semiconductor film 144, and a gateelectrode 148 a (material: tungsten) over the gate insulating film 146.The calculation was based on the assumption that the oxide semiconductorwas i-type and the donor density N_(d) and the intrinsic carrier densityn_(i) were equal.

As the channel length L, two conditions of 50 nm and 500 nm wereadopted. The voltage V_(ds) between the source electrode 142 c and thedrain electrode 142 d was set to 1 V.

Parameters used for the calculation are as follows.

In—Ga—Zn—O-based oxide semiconductor (material of the oxidesemiconductor film) Band gap E_(g): 3.15 eV, electron affinity χ: 4.3eV, relative permittivity: 15, electron mobility: 10 cm²/Vs

Titanium nitride (material of the source electrode and the drainelectrode) Work function φ_(M): 3.9 eV

Hafnium oxide (material of the gate insulating film) Relativepermittivity: 15

Tungsten (material of the gate electrode) Work function φ_(M): 4.9 eV

The results of calculation are shown in FIG. 19. In FIG. 19, thehorizontal axis represents the gate voltage V_(G) (V), and the verticalaxis represents the drain current I_(D) (A) per unit channel width (1μm). In addition, in FIG. 19, a thicker line represents the calculationresults in the case where the channel length L is 500 nm, and a thinnerline represents the calculation results in the case where the channellength L is 50 μm.

The calculation results in FIG. 19 shows that the lowest off-statecurrent per unit channel width (1 μm) is more than or equal to 1×10⁻³⁰(A) and less than or equal to 1×10⁻²⁹ (A) both when the channel length Lis 50 nm and when the channel length L is 500 nm

This application is based on Japanese Patent Application serial no.2010-090267 filed with Japan Patent Office on Apr. 9, 2010, the entirecontents of which are hereby incorporated by reference.

1. A manufacturing method of a semiconductor device comprising the stepsof: forming a first insulating film over a surface; forming a first maskover the first insulating film; performing a slimming process on thefirst mask, so that a second mask is formed; performing an etchingprocess on the first insulating film using the second mask, so that asecond insulating film is formed; forming a first conductive filmcovering the second insulating film; performing a polishing process onthe first conductive film and the second insulating film, so that asecond conductive film and a third insulating film having equalthicknesses are formed; etching the second conductive film, so that asource electrode and a drain electrode which are thinner than the secondconductive film are formed; forming an oxide semiconductor film incontact with the third insulating film, the source electrode, and thedrain electrode; forming a gate insulating film covering the oxidesemiconductor film; and forming a gate electrode in a region which isover the gate insulating film and overlaps with the third insulatingfilm.
 2. The manufacturing method of a semiconductor device according toclaim 1, wherein the source electrode and the drain electrode are formedthinner than the third insulating film by 5 nm or more and 20 nm orless.
 3. The manufacturing method of a semiconductor device according toclaim 1, wherein the third insulating film has a thickness substantiallyequal to a thickness of the second insulating film.
 4. The manufacturingmethod of a semiconductor device according to claim 1, wherein thesurface is a flat surface having a root-mean-square roughness of lessthan or equal to 1 nm
 5. The manufacturing method of a semiconductordevice according to claim 1, wherein the slimming process is performedusing an ashing process using an oxygen radical.
 6. The manufacturingmethod of a semiconductor device according to claim 1, wherein thepolishing process is performed using chemical mechanical polishing. 7.The manufacturing method of a semiconductor device according to claim 1,wherein the first insulating film is formed by sputtering.
 8. Amanufacturing method of a semiconductor device comprising the steps of:forming a first insulating film over a surface; forming a first maskover the first insulating film; performing a slimming process on thefirst mask, so that a second mask is formed; performing an etchingprocess on the first insulating film using the second mask, so that asecond insulating film is formed; forming a first conductive filmcovering the second insulating film; performing a polishing process onthe first conductive film and the second insulating film, so that asecond conductive film and a third insulating film having equalthicknesses are formed; etching the second conductive film and the thirdinsulating film, so that a fourth insulating film whose corner isremoved and a source electrode and a drain electrode which are thinnerthan the second conductive film and the fourth insulating film areformed; forming an oxide semiconductor film in contact with the fourthinsulating film, the source electrode, and the drain electrode; forminga gate insulating film covering the oxide semiconductor film; andforming a gate electrode in a region which is over the gate insulatingfilm and overlaps with the fourth insulating film.
 9. The manufacturingmethod of a semiconductor device according to claim 8, wherein thesource electrode and the drain electrode are formed thinner than thefourth insulating film by 5 nm or more and 20 nm or less.
 10. Themanufacturing method of a semiconductor device according to claim 8,wherein the third insulating film has a thickness substantially equal toa thickness of the second insulating film.
 11. The manufacturing methodof a semiconductor device according to claim 8, wherein the surface is aflat surface having a root-mean-square roughness of less than or equalto 1 nm
 12. The manufacturing method of a semiconductor device accordingto claim 8, wherein the slimming process is performed using an ashingprocess using an oxygen radical.
 13. The manufacturing method of asemiconductor device according to claim 8, wherein the polishing processis performed using chemical mechanical polishing.
 14. The manufacturingmethod of a semiconductor device according to claim 8, wherein the firstinsulating film is formed by sputtering.
 15. A manufacturing method of asemiconductor device comprising the steps of: forming a first transistorincluding a channel formation region, a first gate insulating film overthe channel formation region, a first gate electrode which is over thefirst gate insulating film and overlaps with the channel formationregion, and a first source electrode and a first drain electrode whichare electrically connected to the channel formation region; forming aninterlayer insulating film covering the first transistor; forming afirst insulating film over the interlayer insulating film; forming afirst mask over the first insulating film; performing a slimming processon the first mask, so that a second mask is formed; performing anetching process on the first insulating film using the second mask, sothat a second insulating film is formed; forming a first conductive filmcovering the second insulating film; performing a polishing process onthe first conductive film and the second insulating film, so that asecond conductive film and a third insulating film having equalthicknesses are formed; etching the second conductive film to form asecond source electrode and a second drain electrode which are thinnerthan the second conductive film; forming an oxide semiconductor film incontact with the third insulating film, the second source electrode, andthe second drain electrode; forming a second gate insulating filmcovering the oxide semiconductor film; and forming a second gateelectrode in a region which is over the second gate insulating film andoverlaps with the third insulating film.
 16. The manufacturing method ofa semiconductor device according to claim 15, wherein the second sourceelectrode and the second drain electrode are formed thinner than thethird insulating film by 5 nm or more and 20 nm or less.
 17. Themanufacturing method of a semiconductor device according to claim 15,wherein the third insulating film has a thickness substantially equal toa thickness of the second insulating film.
 18. The manufacturing methodof a semiconductor device according to claim 15, wherein the interlayerinsulating film has a flat surface having a root-mean-square roughnessof less than or equal to 1 nm
 19. The manufacturing method of asemiconductor device according to claim 15, wherein the slimming processis performed using an ashing process using an oxygen radical.
 20. Themanufacturing method of a semiconductor device according to claim 15,wherein the polishing process is performed using chemical mechanicalpolishing.
 21. The manufacturing method of a semiconductor deviceaccording to claim 15, wherein the first insulating film is formed bysputtering.
 22. A manufacturing method of a semiconductor devicecomprising the steps of: forming a first transistor including a channelformation region, a first gate insulating film over the channelformation region, a first gate electrode which is over the first gateinsulating film and overlaps with the channel formation region, and afirst source electrode and a first drain electrode which areelectrically connected to the channel formation region; forming aninterlayer insulating film covering the first transistor; forming afirst insulating film over the interlayer insulating film; forming afirst mask over the first insulating film; performing a slimming processon the first mask, so that a second mask is formed; performing anetching process on the first insulating film using the second mask, sothat a second insulating film is formed; forming a first conductive filmcovering the second insulating film; performing a polishing process onthe first conductive film and the second insulating film, so that asecond conductive film and a third insulating film having equalthicknesses are formed; etching the second conductive film and the thirdinsulating film to form a fourth insulating film whose corner is removedand a second source electrode and a second drain electrode which arethinner than the second conductive film and the fourth insulating film;forming an oxide semiconductor film in contact with the fourthinsulating film, the second source electrode, and the second drainelectrode; forming a second gate insulating film covering the oxidesemiconductor film; and forming a second gate electrode in a regionwhich is over the second gate insulating film and overlaps with thefourth insulating film.
 23. The manufacturing method of a semiconductordevice according to claim 22, wherein the second source electrode andthe second drain electrode are formed thinner than the fourth insulatingfilm by 5 nm or more and 20 nm or less.
 24. The manufacturing method ofa semiconductor device according to claim 22, wherein the thirdinsulating film has a thickness substantially equal to a thickness ofthe second insulating film.
 25. The manufacturing method of asemiconductor device according to claim 22, wherein the interlayerinsulating film has a flat surface having a root-mean-square roughnessof less than or equal to 1 nm
 26. The manufacturing method of asemiconductor device according to claim 22, wherein the slimming processis performed using an ashing process using an oxygen radical.
 27. Themanufacturing method of a semiconductor device according to claim 22,wherein the polishing process is performed using chemical mechanicalpolishing.
 28. The manufacturing method of a semiconductor deviceaccording to claim 22, wherein the first insulating film is formed bysputtering.